All of what you said below this is true. But this I disagree with. If you don’t heal, they die for sure, then you are at a disadvantage, and have less people to heal which makes killing your next target even harder. I don’t WANT to heal stupid people, I just often will if there’s any chance they can make an impact without sacrificing myself. That suicidal player often does do something if they survive longer, and they sometimes survive if you do your job right.
You reduce stupidity by being there. I make stupid mistakes all the time. One game where I was playing Probius, I pushed too far in a solo lane and got cocky. Enemies had rotated to flank me. All of a sudden LiLi comes out of nowhere had apparently left the main group in the middle lane, cleansed me out of a CC, blinded and I was able to zip away, and then she kept body blocking the enemies to take the hits. That LiLi reduced the impact of my stupidity, and for that I got to live, which made the next objective easier to get.
A good healer is often able to anticipate where they are needed to SUPPORT and save. That is absolutely their job. A bad one will just say, well they should just die because they were stupid.
Now weaving in damage, CC, are all part of a supports kit and absolutely critical too in combat WITH your allies. However, bad players often mention killing or mercing in a solo vacuum, and that’s just so wrong for what you do as a support. I would rather heal the stupid person than merc. If I wanted to merc or duel somebody, I would play another role, because that’s not where I’m going to shine as a support.