He left for a "start up" comprised of major talent sourced from his current employer, which is one of the biggest names in the business. He gets to work with his old friends, his old bosses, and probably gets higher pay. In the event that this studio ends up being wildly successful, he also gets to essentially be a founding-level employee. This is the dream for a game dev.
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TBC was the first major expansion, the first time the game had a ton of major improvements, and far more popular than Vanilla. It's going to draw a lot of people back for at least a short while. The same thing is going to happen with WotLK classic.
Current Classic may very well be a flop. It is, after all, Vanilla WoW. Vanilla's main pull was nostalgia and that only carries you so far.