My best guess remains that the popularity of the death knight and the protection warrior in early Wrath pushed a lot of druids and paladins out of tanking. We had more role options than they did, so respeccing to melee, heals, or ranged DPS was a better option than getting yourself or others benched. There were other things going on that probably didn't help much, but at the end of the day it was a numbers game that the bear and paladin simply lost. It was a vivid lesson that design decisions that don't necessarily have much to do with your class or role can wind up having a serious impact on them anyway.
In a perfect world, all of these guilds will fish from the pool of 25-mans shedding players to do mythics themselves, replace tanks who quit, or recruit people who aren't currently doing heroics right now, but the transition to a new expansion has historically been a lot bloodier than that. A lot of 10-man guilds will combine forces or attempt to merge existing raid teams in order to make the 20-man cutoff, and each time this happens two tank slots will vanish. On servers without many (or any) active 25-mans, that seems the more likely possibility than recruitment.
For me, that raises the following questions:
- Will this happen enough to impact the supply of tanks elsewhere in the game, or will so many people be raiding normal and the new heroic that it won't matter?
- Will the new gear model offset this somewhat anyway?
- Because most tanks can only become melee DPS or healers once respecced, is that likely to put population pressure on these roles as well?
Oh, and bears? Might as well build a balance set just in case. Heroic Immerseus ain't that tough to farm.