Miko would be useful even with a nerf to his heals. First of all, no one is intended to sustain combat forever. The death would only be inevitable in the case of combat never ceasing. If someone dies because they refuse to fall back and catch their breath, they SHOULD die, rather than continue to steamroll because there’s nothing to keep them from doing it. If Miko had diminishing returns or a resource to the heal beam, it would add a layer of tactical thinking to Miko. at current, he’s just an overpowered version of a TF2 Medic. It takes literally no skill or thought to do what he does.
Secondly, Miko brings more than just healing to the table, as anyone who’s using Miko’s full kit will tell you. If you nerfed his heal, he wouldn’t suddenly become irrelevant. As he is, his heals are simply too powerful.
You keep ignoring the fact that mid-combat healing is specifically a mitigation tactic, not intended to make a character immortal. And let’s not forget that Miko becomes an equally impossible choice to kill with the right helix choices. It’s not a matter of not enough guns, or not focusing the right targets. It’s simply 2v2 of Miko and anyone vs any two other battleborn should, by the definition of balance, still have a 50/50 chance of ending either way. Sure, there are other factors that swing it one way or another, but at this point there are certain team compositions that are just literal guaranteed victories. That’s not balance.