That’s the thing I think you are missing though
“All roles feel boring in dungeon content”
while that may be true depending on your perspective (I don’t agree with it but I can see why others think that) that alone is not justification for the level of role encroachment that tanks exhibit on healers. Sure completely deleting tank sustain from the game wouldn’t “fix” healers but it would certainly actually allow them to heal more often.
The problem (which is ironically usually leveraged at the healerless ultimate clears) is that the vast majority of people interact with this game via the roulettes and right now tank overreach in the roulettes is excessively high (which is neither here nor there in the front of allowing clears without a particular role as this was possible in SB when we still had a trinity)
Not liking the trinity is fine but whole the game harshly enforces the trinity it’s also not an argument for why one particular role should be de-emphasised in the trinity