Jayne from Mug'thol has an interesting thought over on the forums: he lodges a request to Blizzard to make the current endgame dungeons (BWL, AQ, and Naxx) scalable in the upcoming expansion. We already know a few of the new instances will be scalable-- when you take on the Hellfire Citadel level 60-62 wing at level 70, it'll be harder and have different rewards. So why not, says Jayne, make the current endgame instances scalable as well, so they just as much of a challenge to level 70 players in the expansion as they do to level 60s now. In one sense, Jayne has a good point. Especially in Naxx, an instance that has only been in the game for a relatively short time, scaling it to level 70 might give it a little longer shelf life. And since anyone who's 70 won't even get experience from level 60 mobs, there might not be much pull for players to go back and do the "old" instances.
But personally, I'd rather see the new content be for level 70s and let this current endgame become part of the middle game. Jayne says that having only a small percentage of players able to play this content would extend its life, but I disagree-- I can't wait to get into Naxx with my casual guild in level 65 gear and tear things up. And I'd be very surprised if there wasn't enough content in the expansion to keep players interested all the way until the next patch-- we're not just talking a new instance for level 70s, we're talking almost another half of the existing game. Leaving the current endgame at the level its at will just let more players experience all the hard work and lore the devs have put into it.
Of course, that does kind of invalidate all the hard work current endgame raiders have put into getting their way through Naxx. Thoughts?