They can solve this by giving each class a niche. Normally one would select a defensive niche (like the physical tank, magical tank etc) but that means they have to actually tweak their bosses in order to acomodate that.

If they can't do that, then the only thing left is utility synergies. WAR could give the slash debuff, DRK, PLD, GNB could give a piercing or whatever buff or debuff. That means they would have to match certain classes together.

From the changes in this expansion it seems they didn't like that as well, so they just went "screw it" and made everyone the same. That means people will only take the tanks that do the most dps on raids. So people will complain again.

If they keep it up then in the next expansion they will homogenise the dps and then all tanks will play the same. The only different thing will be skin (like eos and selene).