In regards to the actual concern about the end of the world caused by quest-leveling, I think it's fairly unfounded and here's why:

*You can only do each quest once per character, not counting repeatable quests which are often awful.
*You have (currently) 7 combat classes to level with more on the way.
*If you were to finish all of your quests before you finish leveling all of your classes, you would have no way to level.

What I think will actually happen though is that quest-based leveling will actually become viable and can be used as an alternative to straight up mob grind camps. I don't think either will be the sole way to level or completely obsolete.