Even though the ideal for people is that there be more class variation, for effective balance, the Tanks need to be able to directly or indirectly contribute damage either through their own personal output or being able to supplement the output of others that would be equivalent. Clearly PLD did none of this.

It's been stated before but PLD's mitigation is another case of Big-Numbers syndrome wherein people judge its capabilities at face value.
40% Sentinel? Wow that's better than everything WAR and DRK have! PLD BEST TANK. Hallowed Ground plain invincibility?! OPOPOP.
People unfortunately fail to see the full context of the abilities as well as how they actually play out in current content design end up setting themselves up for disappointment.

Even in their respective tank stances a Warrior would mitigate AND contribute more damage than a PLD.

If you wanted the reverse, you'd have to make PLD mitigate something like x5 better than the other two, whilst nerfing its personal output to borderline pure auto-attacks and/or further buffing its support capabilities that directly supplement damage from other classes. Content would still have to be tuned so that the weakest tank mitigation-wise would be able to tank it fine(in this scenario WAR/DRK).

Doing so however opens another whole can of worms wherein PLD would find great difficulty in soloing any content, and his contribution be completely reliant on whether or not his/her party members choose to even utilize it. If for example the PLD is stuck in a dungeon run wherein his DPS have died on their keyboards and the healer desires only to heal, then it is simply impossible to even clear said dungeon whereas if a WAR/DRK whom are able to directly utilize their own contribution have a better chance to clear the dungeon with said DPS and healer.

Honestly an borderline invincible tank is probably going to make both the healers bored as well as the invincible tank. But that's my opinion, perhaps people enjoy doing nothing while they play.

I mean, if you really wanted it, and it fits fine with the endgame balance then maybe, but just be aware of the consequences of the other end of the spectrum.

But seriously, what the frisbies is going on with Shadow Wall? It's so half baked compared to Vengeance and Sentinel, it's just plain worse than the two with no redeeming factor for itself!