Unironically I think delves could easily be S tier content with some tweaking. I actually enjoyed them (as a tank), and if there existed a level of delve that awarded myth track gear in vault, I would certainly push to do it.
I think balancing them with a variable player count is difficult, which clearly we saw this season, but I do think there’s a lot of potential to create a competing game mode to M+ for a large portion of the player base.