I mean, Rian Johnson kinda set up a Rey heel turn in The Last Jedi, when Luke admonishes her for not even hesitating to dive right into the dark side cave. He also tells her how her sheer power scares him and that he's only felt that way around one other person (presumably Palpatine). That said, Rey's just too optimistic and hopeful to go bad.
I reckon Ben's redemption was even more unearned than his grandfather's was. Anakin at least saved his son by sacrificing himself. It doesn't excuse all the horrible things he did as Vader, but it's better than Ben just kind of deciding he doesn't want to be bad anymore after Rey defeats him and then saves his life.
Having Ben turn back to the light to become a true Skywalker hero while Rey goes full-on Palpatine would've been interesting, but I think it would have been difficult to pull off.