I’d say you’re oversimplifying things. The point is the exact opposite of “Leia knew and didn’t care.” It’s the fact that she DID care so much as to return Rey the Skywalker lightsaber and literally say “you are family to me” despite what she’d seen potentially happening in the future. It’s the exact inversion of what Anakin does. Her placing so much hope in Rey is what gives her the strength to push back against that dark fate. It’s extremely powerful stuff.

Whereas the way we currently have it, Leia realizes there is a struggle Rey is going through but may not perceive the exact depth to it. It’s clear that she cares, but who’s to say she wouldn’t be quite disturbed at the idea of Rey becoming Empress of the galaxy? Would her behavior to Rey have changed if she knew the truth of her struggle?

It may not be a big reveal to the audience, but it is to Rey since she is living with that uncertainty of whether or not Leia would judge her for all this. It’s a character development moment, not something to make people gasp.