You misunderstand, I think there's plenty of things we could learn from WOW. I just want to know, in that fight where the healers had very little healing to do, not because of a lack of damage per se, but because they literally could not heal thanks to the fight's mechanics, what did those healers do to fill the downtime? I'd imagine that it'd be damage, which is just what we have here, in which case the two games are very alike. The difference was that there was more damage to heal back then in WOW, but we might not be able to get away with raising healing required here as much because WOW has something we don't: responsiveness
I think the analogy falls apart a bit when you consider that the four healers are part of the same role, therefore they have to be related. Probably makes more sense to say that each role is a hobby, like 'sports' or 'video games' or 'reading books', and within that category you have different things. Like the four healers can be four different sports, and some sports involve a ball, some don't (running/highjump/longjump), but at the end of the day they're all linked in that they require some amount of physical exertion, like how healers have 'healing' as their core. And regardless of the sport, all of them have a fair amount of depth to 'master' said sport. Pretty sure most people agree that the four healers should feel different in their damage, there's just disagreement on why one should be left as it is