This decrease in new VGM fans has not become apparent to my eyes, yet I haven't been around the online communities since the beginning to witness the progression as a whole, but I'll take your words about it. I understand the possible causes that were mentioned so far and I agree it would be harder to become a huge fan of game music these days. There are some other points that I believe might help to explain the phenomenon, and I'll also take the opportunity to respond to a couple of opinions that were shared so far since I was shocked by some!
First, to become a fan of game music, being a gamer helps a lot! You get the chance of being exposed to it directly which makes you develop an affinity for the sound, structure or how it's connected to what happens in-game. Then, listening to VGM outside of its primary context becomes easier than for someone that never played to games, or that actually hates them, and that's one of the reasons their "door" is usually shut tight. In theory, new gamers should also fall under this concept, since they are in contact with VGM and thus have the opportunity of enjoying it. So why isn't it working as good today? Many reasons obviously, like those you already mentioned. The music today doesn't sound as unique in huge part because of the more advanced technology and the realistic sound, that's for sure. For many current gamers, appreciating the music in such games would thus lead them to become more of a music fan than a VGM fan I guess, in relation to considering "VGM" as the more retro/electronic style here. Is that necessarily a bad thing? If the music sucks, I'd say "yes". If not, even if it sounds like mainstream music, it's not a problem, or at least not on that level, because what's most important is that the person enjoys the music, whatever genre it is (and there's so many genres and tastes, there's something for everyone!). Actually, I consider myself a fan of music first, who also happens to appreciate game music. When I hear a good rock song from a game, and know it sounds like a real rock tune from the radio, I still like it, and don't try to reject what's there because it's not like "VGM". And I enjoy the song from the radio too. I believe this is one of the big problems with some VGM fans; they believe that in order to be a real fan, you have to listen exclusively to VGM, or at least have it at the top of your list, and that you must dig the old or unique style, because the rest shouldn't even be considered to be VGM. I can't deny that this is being a fan of it, but it's being "hardcore". To me, VGM is any music that is found within a game (you can discard licensed songs though), even if it's not mainly electronic-based or ultra-melodic. However, I'm not blind and I know there are certain styles that are specific to this category of music, and by using the term "VGM", those styles get a larger part of the definition, but still, not all of it.
I guess this topic relates to one that was discussed here recently, about how much VGM is in your playlist compared to non-game music. I'm certain many of the "first-generation" harcore VGM fans don't listen to much of anything outside of it. This doesn't mean it's a bad thing. However, it becomes bad when they start to criticize how people can't see how great VGM is and that it's only because of MTV and that those people can't tolerate instrumental music (and that the only instrumental categories are VGM, classical and that shitty jazz song they heard at their uncle's house 5 years ago). It's ironic, they shout at something they also do: closing eyes on something unknown. A testimony of ignorance with a dose of stubborness! Sure, it's possible their favorite music is from games, but bringing forth that everything else sucks is quite radical, especially when it's impossible that they heard all music that exists in life. While I won't cry, I still find it's sad for them...
Back in the old gaming days, Internet wasn't really around. So, for geeky gamers, their main source for music was from video games. Growing up, some lost this interest, while it became more solid for others, like the VGM fans here. There are many factors implied in this enjoyability, but the nostalgia and familiarity with the styles and sounds, as well as the security they provide, sure do play a big role. Also, not having been exposed to other music they could like (either because they didn't pay attention to radio and bought CDs, or because they didn't dig what they were hearing), made them go deeper into VGM and ignore the rest even more. Today, with Internet being fully developed, it's so easier to explore music, it's unbelievable! Legal or not, it gives the possibility to people to discover things they would have never thought existed. So, the geeky kid who likes video games is not as limited as a decade and more ago to which music he is exposed to and what he can learn to enjoy. Does this mean he won't like VGM? Absolutely not, but he sure will have more chances to grow up as a music lover (game music is not excluded) rather than only a VGM fan (excludes other music categories, or at least a large part). I believe this is one of the reasons why there won't be as much VGM(-only) fans in the future. I guess it's bad for the state of the retro tunes, but what is really good should be timeless classics, and the creation of new material in the same style (while not having to be chiptunes) shall be welcome as long as it's the trend (because that's how the industry goes), even if the fans want more of it. Different styles should also be welcome, for the sake of diversity, even if they are not as creative as the ones back then or some today. And if you believe the only thing time and development brought to VGM is orchestral snooze-fest, check again! However, if what you seek is only the classic VGM style with a different sound (and this also applies for VGM fans who are looking for non-game tunes), I agree it's hard to be pleased and it's even deceiving, but on the other hand, there are people existing aside from you who can actually enjoy other styles, so this situation is not completely bad (it doesn't always have to be "all" or "nothing"). Broadening horizons is a wonderful thing, but keeping a certain focus on the things we prefer is also essential. Also, about what Jodo Kast mentioned, I agree that in order to be a real VGM fan, you have to be independant from the generic preconceived ideas of society (the mainstream), since this music can be considered as being poor or in association with VG/anime nerds (which is in part true, but I'm trying to dissociate myself from it as much as I can). Actually, this should be applied to everything in life in my opinion, as long as it follows a certain logic and ethic. But I think some people reject everything too fast, be it those who don't listen to VGM and the VGM fans who don't listen to other music. It's as if all things have to be categorized as being "black" or "white". "I won't play that music because nobody does, and it sucks" vs. "I won't play that music because everybody does, and it sucks". Ignorance also comes in here; you don't need to be 100% into something to appreciate parts of it. Also, it's funny to note that there is also a mainstream within VGM.
On to answering to what recent game music would captivate someone enough to generate a devotion to VGM, I'm also not very certain. I have to admit that some of my tastes aren't popular ones in comparison to other VGM fans around, so even if I'd mention a certain soundtrack that would actually lead me toward a conversion, it might not apply to others. From what I observed though, I guess RPG still fetch the most people, especially the anime fans. FFX-XII, Atelier, etc. Girls are actually very much falling under that category as far as I can tell, not that it's very pertinent. On the Western side, the main entry points seem to be popular games, in the like of strategy and shooters. But I'm wondering how many actually listen to the music after they have downloaded it (OSTs usually don't exist). Again, this question depends on if people have to like VGM because it's VGM or because they like the "M" part; the first would be more likely to hook then on, but the second can still have them enjoy VGM. Also, some soundtracks these days might seem to be not as good as old ones, but there is the notion of comparison and look back in play, so in 10 years, they might seem better than today (or worse, hehe).
I'll finish by commenting on what Carl said, because it irritates me to a great extent. I'd like to mention it's nothing personal, but implying that "Converting to game music", which should be the same thing as becoming a VGM fan if we're under the same definition, is to order an official CD album from overseas, it's ridiculous! Sure, I understand your point and agree it's a good action to encourage the VGM industry, but at the basis, it has nothing to do with being able to enjoy game music. I guess it falls down to the fact we must have a different point of view on VGM. From your comment, I would say you view it as a collector, that VGM is mainly about the pressed CDs and other official releases. Several websites are built this way too, focusing on a database of albums. On the other hand, I consider the starting point of VGM to be from the games themselves. You can appreciate the music while playing the game it was composed for, and then sure, you can also listen to it outside of context, by buying original/arranged soundtracks or by doing/finding a custom-made soundtrack (i.e. game rip). You can listen to the soundtrack directly without having played to the game too, it doesn't matter, but it's just that game music goes with games for its origin. In that idea, the fact there are so many games that don't have soundtrack CDs makes it clear to me that the games themselves should be the source point and not albums (these come after, a bonus of some sort). So, you can develop an interest for game music without having to buy an album CD, although doing this doesn't goes against it and is more moral when an OST exists (and is easily purchasable). That's what I perceived from yout comment, but if I got you wrong, please tell me.