Would I still tank? Probably.

Would I only do it in raid groups? yes.

I really think you'd see a sharp decline in tanks if this was the case. The reason this game has the amount of tanks it does is that tanking is the closest thing to a hybrid role in the game. Being able to perform both sides of your job at a high level takes practice and understanding of fights. The challenge of doing both these things at the same time is what lures a lot of us to the job role.