Gotta love Zane (no homo)!
Only the true ruler to the Twili has the power to destroy the Twilight Mirror utterly. Zant could only shatter it to pieces while Midna destroyed it, so it solidifies her claim as the true ruler.
I think that's the only reason why they had her break the mirror.
Anyways, I think a better question is the whole Ganondorf/Zant-neck-snapper moment. I have my own theory but I'm not particularly married to it. I think Zant actually snapped his own neck, and as a result, Ganondorf couldn't exist in the world without Twilight being as though Zant was the connection between him and light world for damn near the entire game. Feel free to debunk this, because I really have no idea what the hell really happened. Also, I think the notion of Zant having the power to "kill" Ganondorf (he cannot die!) doesn't make sense. Ganondorf was a god to Zant, and besides, only a certain Master Sword and guy named Link who just happens to be chosen by the Gods as well could ever "defeat" Ganondorf; as evidenced further by even the Sages being unable to kill him.
Anyways, I think we'll get another game with TP Link. I think it's a little too early to put him to rest... he didn't even get an title (i.e. Hero of Time; Hero of Winds).