The issue with entirely "soft enrages" like you described is the DPS themselves instantly have less value. In fact, it'd be better to bring a third healer or even tank simply because that gives you a larger pool to work from. Couple that with the fact healer kits are already comically overpowered and they'd have to design the end mechanic AoE spam in such a way that the boss just keeps dealing increasingly higher damage ala Brute Justice. At which point, what's the difference? After a certain number of stacks, Brute Justice simply deals too much damage for the healers to handle, thereby being a hard enrage. While a fun idea I wish they'd use more often, the novelty would inevitably wear off when every single boss has to follow the same format to ensure fights don't carry on forever.