There would not be enough tanks who would commit to tank in role queue if two were required, but there are enough tanks willing to flex Also, with bans coming soon, I think the meta will naturally begin to favor two tank team comps again.
They started with 2-2-2 RQ, OP. That happened in December.
The reason that RQ’s 1-2-2 nowadays is because 2-2-2 RQ’s not sustainable. That’s no reason for OQ modes to stay 5v5, though, so it makes sense to upgrade it to 6v6 (but with a tank cap to avoid gross tankfests). Those folks in 5v5 can enjoy their RQ.