I get what you are saying, but in 1980, Yoda saying “Once you start down the dark path, forever will it dominate your destiny, consume you it will, as it did Obi Wans apprentice.”the dark side is not some sort of mind control virus. It is simply the shortcut, the easy way out.
Yoda's warning was not that once you dip your toes in the dark side it will haunt you and eventually you will turn, it is that once you compromise and take the easy route you will be tempted to do so again because that is human (and alien) nature.
Someone who has killed will find killing again easier than a first timer. Someone who has resorted to violence in the past to deal with problems will likely do so again. It doesn't mean they cannot change, some very violent people can be re-formed, it just takes effort to avoid temptation to take negative actions.
Anakin gave in the dark to slay the tuskens in anger as the easy way to deal with them (rather than say, try to get authorities involved or some other non violent action) but continued on to be a "good" man in the light side, even a hero, but he had learned that action can remove a problem immediately rather than waiting for the proper processes to deal with it and potentially allow future victims to fall at the hands of enemies. He decided with Dooku that just killing him would be better than taking him prisoner, again the easy way out. But that was Aniakin, he was shaped by the fact he saw people being victimised through inaction on the part of those in authority, he grew up a slave after all, so his worldview and experiences/losses made him particularly vulnerable to that way of thinking.
Given that Luke was not one to seek power the way Anakin did he likely would not have been as tempted by such shortcuts, though he did have the weakness Anakin had of being protective of his loved ones, so that is where his flaws should have been shown to be exploited from. His protectiveness of Kylo should have been what allowed Kylo to go on to become a Sith/Ren/whatever.
That was taken very literally. Once you use anger and hate to gain power or the upper hand, you will always be tempted to keep doing it….
I guess you can argue, Luke looked down the precipice, and backed away that the last crucial moment….I always found it interesting idea that all the killing Luke did in ROTJ was going to corrupt him….after all, the only other Jedi we saw never killed anyone at that point, he even let himself be killed to save others…..
We start ROTJ, with a traumatized Luke using what we assume to be a dark power (force choke) in the very first scene. Something we only saw Vader do before.
Then he pretty much killed several dozen people in the rescue attempt….not to mention all the people who died on the Death Star.
Its obviously much easier to speculate on this after 30 years, and countless hours of new content.
I really would have loved to see Luke struggle with the Dark Side more, but I see both sides and why some people will always want his to be their hero.