I'm not sure what they will charge for a 90, but chances are it will be a HUGE money generator. People have payed for this service in the past. At least Blizzard doing it puts it in a controlled environment. But then, it never made sense to me that Blood Elves/Draenei didn't exit their area at 58, Death Knight didn't finish their area at 68, and that Worgen/Goblins didn't have the option to exit their area at whatever level you needed to be to do Cataclysm. The same goes for Pandaria. Last time I quit so many months ago was because I got burned out on the trek to 85 and Mists leveling wasn't THAT enthralling. Blizz wasn't their subscribers back, and they aren't going to get them by making them grind through the expansions that made them quit in the first place.

I think people are confusing the Time Travel aspect of the expansion as well. The story isn't going to be about going back and killing your own Grandpa, or Thrall accidentally breaking up Durotan and Draka and having to get them back together on prom night and then take his delorean home. It's like JJ Abrams' Star Trek. It involved time travel, but it wasn't a time travel story. In this case, the player is Old Spock. We know how things went down in our timeline, and this new one has some weird stuff going on in it, but changing events doesn't change our past.

Personally, I'm betting on them setting up the next expansion as covering the First War, either as a complete reboot with a new engine, or a further item/level crunch. That or they're setting up Warcraft 4.