It is not pointless to ask for an adjustment... It is just the TYPE of adjustment you want is pointless. You would have to break the game to give PLD enough mitigation to make it worth brining over higher DPSing tanks.
PLD needs much more utility. Honestly I feel like PLD should bring more healing related buffs. It would be nice if they find some way that the mere inclusion of a PLD to your party comp in some way buffs healers ability to heal THE WHOLE PARTY.
Also your idea of making a tank SO tanky that healers can DPS goes against the obvious design that SE is looking to force healers (RIP healer ACC).