Except that you create a game where there is only one option for tanking, which is bad. The entire point of having multiple classes is so that you actually have options for playing a given role. If the devs turned WAR into a DPS, like so many facepalm-worthy Gladiator fanbois on this forum seem to want, then tanking would stagnate because there's only a single option. It would be like finding out that there are only 4 classes in the game: PLD, BLM, WHM, and MNK. If you want to tank but think that PLD is boring as hell, you're either screwed or bored. If you want to rDPS but don't like BLM, you're either unhappy or screwed. If you want to mDPS but don't like punching things, you either learn to cope or you're screwed. If you want to heal but don't like the fact that you don't have any support tools aside from direct healing, you either learn to like have 8 abilities that all do the same thing or you're screwed.
Class choice is one of the few major choices that players get concerning their character. You choose between tanking as a WAR or tanking a PLD, and the choice matters because they tank *entirely differently*. Even if one tanks slightly worse than the other, it's still a choice that matters and it *needs* to be there because only having one way to do a role gets boring *really* fast.