I mean did you not see #pulltheripcord prior to SL launch? There's a reason why there was so much discussion over #pulltheripcord.
But honestly, if your perspective is from a "casual" gameplay then covenant choices shouldn't matter. Isn't this really the bigger issue? Either you're a casual player and performance really doesn't matter that much or you're a semi-serious (or more serious/competitive) player and these choices matter.
And the systems built around end-game progression are similar. For instance, why does a "casual" player need a ilv 235 legendary? Is a 190 or even a 210 not enough?
Eh... That's not entirely true. Experimentation has been done before (see Crucible of Storms) and even in the current raid you could bring in an extra tank if you wanted to... In fact, didn't we see exactly that? In some special cases where only a group of DKs took down certain bosses? Or how about back in BFA where Heroic Ny'alotha was fully cleared by 29 fire mages (+1 DH). So clearly it's not a "set in stone you must have 2 tanks, X healers and Y DPS".