This problem with this line of thinking is that reducing the number of tanks in-game is bad in the long run.
I'll admit that the current approach to WAR is iffy because of the stances. Where most games would allow the WAR to grow depending on which stance they prefer to play as, XIV is hindered on top of that by the armoury system. Frankly, if we're to keep the armoury system they need to chose a role and go with it. I'd prefer it to be tank just because we do need something more than just PLD for that role.
Now if the devs were to introduce SAM or BST as tanks to take up the spot on the tank roster occupied by WAR, I might be willing to support making WAR a full on DPS.
You're going by the logic that divided people between "blood" tanking and Utsusemi tanking, which is fallacious given that the tank role is pretty much straightforward no matter how you slice it. The main difference between tanks should be along the lines of flavor rather than mechanics. That's what distinguishes the guy with a sword and shield from the guy with holy magic and a shield from the guy with a two-hander. You stray from that and you start forcing people to cherry pick jobs for their group comps.
8 is an iffy number for a full party, mostly due to what it allows in a group comp. Ideally that could mean two tanks, two healers, a bard, and space for 3 DPS. Limiting groups to one tank limits the developers as far as what can be done with encounters that require a full party. Sure, stuff like behest wouldn't require much, but a dungeon boss with mechanics might require an additional tank either for adds or taunting off each other or something to make the encounter interesting.