But what will you do if you end with WAR or DRK in a the Duty/Raid Finder ?
That's the problem with Yoshi-P's view of PLD. A tank can't focus on mitigation, since mitigation is the basis of the role, unless you really push it to the extreme. Make PLD sturdy enough that it doesn't need a healer, and suddenly, it will stand out. You can decrease its DPS to abysmal level as a tradeoff, but your party will have 5 DPS instead of 4 to compensate.
And frankly, DRK and PLD could have been close enough to each other without basically recycling Shield Oath and Rampart.
A suggestion I made a long time ago about DRK is to have its tank stance be a reactive leech. Each time you're hit, you reflect back 20% of the damage (Capped by DRK's HP max), and leech this amount as HP. Basically, you'd still only "suffers' 80% of the damage, like Shield Oath, but you'd have to stay high enough on HP so that the hit doesn't kill you before the leech effect procs. And it would also be your main source of damage, while your WS would focus on enmity generation and debufs.
Isn't the WoW playerbase way higer than that ? Being an expansion, you would say that every active player would buy it, especially if WoW's expansion are not tied to each other and you can jump into Legion right from the start.