I was willing to give you a like until you brought up the "success of a game" as a metric by which to judge it's merit. Warhammer 40k is wildly popular in my area.The point is that very, very few people like playing healbots. Even I who played one now and then (I still prefer tank) in MMORPG never ever wanted to play one in TTRPG, and absolutely no-one in our 20+ people group wants to play one. Everyone prefers having characters who can do more things than heal all the time. Even in the loooong years of BECMI then AD&D, people who played clerics mostly wanted cool gods and the associated roleplay, not the "healbot role". And let's not forget that people NOT wanting to heal is the reason for which clerics have almost been one of the most powerful classes (which is also why some people hated clerics who did not want to heal, some people thought they got all the good things and none of the bad).
Yes, it is, 4e then 5e made it entirely optional, and I think it's a good thing. You can still be one and contribute in other ways, which is cool, because, again, there are very very few people who want that role anyway.
And it's not a good thing, it wanting a healbot which is a completely alien way, because IT DOES NOT EXIST IN THE GENRE. And that's a really important point for me, by the way, that way of playing it totally technical. There are non-combat healers in the genre, usually for low magic settings, and for a good reason, hit points are totally abstract and represent many things and thinking that a healer heals the loss of confidence, skill, luck, divine blessing, etc. is something that self-respecting writers don't put in their books/movies because it would break the suspension of disbelief (and make people dislike their production). 4e then 5e acknowledged the fact that it's all abstract anyway and we are just trying to simulate a genre in which no hit/effect has any immediate or lasting impact unless it downs you (which is dramatic, and therefore cool), and even that can be extremely temporary because otherwise it's not fun to play.
And still making it way better, see the success of 5e...
Very few people will seriously defend the game is good.
EDIT: I apparently failed to make clear that "the game" in this case is Warhammer 40k, not D&D.