Raiders throughout the lifespan of WoW has shown that scheduling ahead to play together is always going to be possible. There are many people who can do that, so the player base would not even come close to dying out if teams would be a thing again.
TBC Classic will show a change from how people connect in PvP and how rating is progressed compared to retail if they go with the old system, and better social atmospheres overall.
But that doesn’t mean teams needs to become a thing again. Honestly, teams needs realm communities, so forming teams now wouldn’t really fit into the game the way it is now.
^ That goes over basically everything.
^ That goes over what a solo queue would require, for the most part.