I do think it's correct that difficulty shouldn't correlate to damage; a hypothetical MCH which is exactly the same but you have to press an extra button every five seconds or else take a damage down shouldn't get extra damage for its trouble but instead not have the trouble. What SHOULD be worth extra damage is fragility, ie the capacity to lose damage to factors outside your control. For melee characters that (used to be) a combination of short range and positional requirements; for spellcasters it's the conflict between attacking and moving. Ranged physical dps could on the one hand be "buffed" by the existence of bosses which give melee and caster characters legitimate positioning and movement problems, or with their own slightly inconvenient or inflexible mechanics (occasional cast times or self roots, attacks that do LESS damage with proximity, etc).