Again, black and white examples and theories when actual raids are not so simple and encounter margins have a wider range of tuning and balance. How much experience do you even have with the content? Or are you just mindlessly parroting points made within a different context?

Because PLDs have objectively better physical mitigation under the right circumstances, you can structure damage mechanics to reward that. If you had a physical tank buster on a 30 second timer and other mechanics designed to lock up dCDs like Shadowskin, Vengeance, Thrill, Rampart, etc., then the PLD could stay in Sword Oath and mitigate with Sheltron + RoH while WAR would be forced to spend their stacks on IB rather than FC and DRKs would be forced to sit in Grit. You just need to properly tune the damage to wedge into that mitigation gap between PLD and WAR / DRK.

Raid design currently is designed to reward DRK's mitigation. I don't know why it's impossible for you to see how the same couldn't happen to PLD. Because DRKs have DA + DM for the magical-damage based tank busters, they can save their other CDs like Shadowskin to negate the mitigation loss from dropping Grit. In the early weeks of A1S, PLDs had to stack Rampart and Sentinel to mitigate the tank buster. DRKs could Shadow Wall + DA + DM and achieve higher mitigation while saving Shadowskin for other uses. The raid design rewarded DRKs with specific tuning and balance.

Because of how unique Hallowed Ground is, you could easily design mechanics to react with it. Like I mentioned earlier, if HG acted as a true immunity, you could use it to absorb all the royal pentacles in A4S with one PLD and HG and save your party from the need to sacrifice three people. Just look at HG in T13. It allowed you just to eat an Ahk Morn without sharing. And, on the point of Ahk Morn, its design also gave value to cover in a more meaningful way.

Viewing the issue almost exclusively from the viewpoint of healer DPS shows you don't understand the scope of the issue or the nuance of it. Being tankier can have other influences outside of healer DPS uptime.

There are plenty of ways to design content to reward varying eHP values while still retaining the viability and value of tanks with less eHP but more damage. Just because you don't have the experience, intellect, or vision to see it doesn't mean it's impossible. So, when SE says that they're going to try to balance the classes through the content, while I don't have much faith in their competency, the idea is perfectly viable.