But we aren’t necessarily talking about Rey’s blood specifically, is the thing. I’m shifting the meaning of the lesson to be that, for all the power in the Skywalker bloodline and Rey’s, it was all for naught because their spirits weren’t strong enough to overcome the dark inclinations of the Force. Leia saw somebody who would have the spirit to do such a thing.
It’s also cool if what Luke tells Rey to do is what she tells Palpatine (“I’ve come to end you”). Then Palpatine nonchalantly is like - “Yeah, that’s what I always wanted.” It immediately dispels the pep talk that he gave her, and returns the stakes to the scene.