They can balance anything to be like anything. No one is assuming they "can't". But that's not the way they've been designing content, nor does it seem to be the way they want to design the content.
If for example Hallowed ground prevented the need to sacrifice three players like you suggested, allowing for the usage of Lb3 for damage. Who do you think raiders will take?
Players will always take the best/ easiest/ path of least resistance for content.
Somewhere someone is going to get shafted unless all tanks perform equally well MT or OT. They don't, and unless things are changed through utility, dps, and mitigation, they won't.
Alexander doesn't make bringing Pld / DRK comp a good idea because you'd be less optimal to not bring a Warrior to anything with two tank slots. You already said it yourself, they have the tools. Drk and Pld have tools locked behind being an MT. One of those tools Warrior gets to use whenever they want. If DRK or PLD weren't slated as 100% having to MT, and warrior actually had to compete for a slot with them, there wouldn't be an issue.