Well first reaction is probably it's off-tank who gets benched/dropped as majority thinks need Winston or Rein, but when all current 8 (and possibly more?) heroes are meant to be "Main Tanks" in OW2 I don't think there will be a single player who can master all of them or 5-6 of them that team would be willing to go whole season with 1 tank.
To be fair it won't affect that many players as there is 54 Tank players in the League atm, which is basically 14 "extra" for current needs and I'm pretty sure most teams end up going with 2 tanks (assuming max team size might be changed to 10 though) maybe few brave teams go with 1. Sure 20 people effectively won't be playing, but can someone name a tank player who is excellent Rein, Winston, Orisa, Dva, Zarya, Hog, Ball, Sigma (in their current builds)? No need to think who could be as we don't even know, what stuff will be changed for some of them.