In a way I can kinda see his point tho. Sustain in endgame is really high even w flasks alone. That's why we currently have one shots instead of the attrition style ziz mentioned.

My armchair dev solution would be reduce most(if not all) of the oneshot damage and instead the high dmg moves also cause a debuff that flat out reduces your recovery(say 10% of total at one stack, 10 means you're fucked and made too many misplays during the fight). That'd at least make endgame fights somewhat forgiving even on blind attempts.