Erakir I think your post was really interesting and I surprised myself by reading through the whole thing. It made me think about some things I otherwise might not have. I feel like the answer to there not being content designed to feature HoTs is a simple one; there is, dungeons.

I know this isn't the same thing as EX primals and Savage but there is no denying that in a dungeon run HoTs reign supreme. WHM isn't the undisputed dungeon crawl healer just because of Holy spam (though that helps keep them out in front of Diurnal AST) but because the 1k+ potency of Regen for a measly MP cost means spending almost no GCDs on healing and hardly any MP in many scenarios.

Also, in response to the concept of "you might get two WHMs in DF" I feel like any content you aren't going to want double WHM in you should be making a PF for. PF gives you complete control of who joins. It's the same thing as wanting a RDM and a BRD for progression in a new instance, you make a PF so you don't find yourself with BLM/MNK/SAM/MCH as your DPS and recieve almost no support. Can you clear new content that way? Sure, probably. Is it harder than having a backup Rez buddy and Foes? Yes, undoubtably.