I'd just to like to make it clear again, because I feel as if my point is getting misinterpreted as "Dungeons should be easy and healers should have nothing to do" No, my point is that they're currently designed to be easy, so it's not a surprise that you can run it without a healer or tank pretty easily. Saying healer isn't useful in dungeon content! is not a argument for why healer is poorly designed, it's that every job tanks included feels bored during a dungeon, your point about tanks not really needing to need mitigation for tank busters is something I don't fully disagree with at least in later dungeons, apart of that has to do with item level synch (this is a massive reason why dungeons feel super easy, that never gets brought up) and apart of that does have to do with tanks passive built in 20% mitigation and higher hp.
I'm not a fan of the strict trinity system in ff14 at the same time, it's very basic and boring to have dps "do damage no utility" healer "heal" tank "aggro" the problem is if you want the "trinity fantasy" it ends up hurting job design and variety such as paladin's having access to some healing skills wouldn't be allowed in a system that was strict enough and enforced the trinity system 100%, in a strict Trinity formular every tank and healer bring the same thing on the table because they become designed to only do one thing. I do not like strict trinity systems, I enjoy more adaptive trinity systems.
My personal thoughts are that dungeons should be more difficult but not to a drastic point where it makes new players struggle, I think a part of that is fixing item level synching, toning down some mitigation values on tanks (and things like bloodwhetting aoe healing the warrior to full every 25s), but at the same time I also think runs without healers or tanks In casual content is fine to a extent.