I think Luke’s temptation is there to show that you never stop needing to resist the dark side, no matter who you are. And Leia’s decision to give up on Ben boils down to a Wrath of Khan style equation about personal needs vs. the greater good. She gives up on Ben because he’s going to burn the galaxy down. It’s a counterpoint to ROTJ’s message to be sure, but depending on how IX goes I don’t think it necessarily undermines it.
Darth Vader did burn the galaxy down, but that didn’t make Luke give up on him. Yet, here he is contemplating to slice his nephew in half before he’s so much as started a bonfire, which essentially pushes him over the edge. By the end of the film they’ve all given up on him. If that doesn’t undermine the OT message, what does?