LFR Lei Shen sort of did this, and folks simply dropped group on a large scale instead of bothering with it.Blizzard should offer queuable content that ramps up in difficulty the further you go in progress, and only puts people together that already cleared the previous layers of content.
It's a niche game to begin with, that just so happened to have lightning strike for it in terms of subs. If anything, a playerbase shrink would probably be the best thing for the game, really. Deciding to focus on a specific type of player, (whatever sort of player they wind up wanting to appeal to), would likely make for better game content than the 'try to please everyone' angle that's being played now.Something along those lines is the -only- sustainable system. Without it WoW is going to become a niche game only played by a very small amount of people as time goes on