On the idea of a passive healer- you wouldn't be able to let off of them. They would require constant attention. (Perhaps they have passive healing for every several seconds they are not hit, and they try to maintain distance and evasion when trying to heal).
Thing is, there are already characters that demand constant attention, and some people find them annoying. If you let off zoners, they spam with projectiles. If you let off Cloud, he'll charge his limit and have an advantage the next time he's up close. If you let off Hero, he'll reach into his magic bag and do something
unstable. But all of these things have ways of losing their utility or outright failing during the match. Projectiles can miss and are no longer worthwhile at certain % levels. Cloud has to land his limit break before he loses it. The Hero's command select is, by design, unreliable and requires resource management.
Passive recovery sounds like a mess on paper. For the player using it, there's no advantage to
not healing, meaning there's no reason not to spend as much time avoiding the opponent as possible, which is something Smash makes easier than other fighting games through movement options, large stages, free-for-all battles, and the fact that the entire game revolves around creating distance by launching your opponent away. For the player fighting against the healer, they have to keep pressing on against a turtling opponent or else their hard work will go to waste. There's no real decision-making here, the counterplay is something they were probably going to try anyway. And what happens when these characters get into a mirror match?
I just think there are so many different ways for it to work, that saying it flatly doesn't work is just... well, as I already said, unimaginative.
As far as 'not an interesting concept,' I mean... have you seen Fire Emblem?
fire emblem bad, yes, I'm aware.
Sorry for being unimaginative? Not saying it's literally impossible to make something out of the concept, but if we're just going to default to "Sakurai could probably figure something out" then I don't even see the point in having a discussion about its merits.