There’s a difference between being a “legendary hero” in the context of our real life view of the OT as a story told on film and being a legendary hero in universe and all that comes with that meaning (though of course the two are metatextually related).

The ST doesn’t “retcon” the OT or make it wrong or whatever (ruin it, I guess?). The OT is still the OT and will always be the self contained story that it is and nothing that comes after can change that. But you admit Luke ended his hero’s journey there, so what is the ST supposed to do then? Just preserve Luke in a glass case of “perfect mythic hero”?

The ST does exactly what a sequel should do. In many ways this is very similar to the way Empire re-contextualizes the original Star Wars.

The original Star Wars is my all time favorite movie. But it’s not my favorite movie as the first film of a saga or as the fourth. It’s my favorite movie as a standalone, modern mythic adventure fairy tale. I can still watch it and love it in this way, even though Empire necessarily complicates the characters and the universe (and the myth) beyond what we see in the original. Empire can’t take anything away from Star Wars, it can only add to it. That’s what the ST does to the OT.