A high Skilled Champion shouldnt have 50%+ Winrate in low Elo , which suggest she is completly broken
I think we need to clarify few things about skilled champs. There are two attributes, skill floor(how hard it is to first time champ) and skill celling(how hard is it to get maximum value out of that champ)
Riven, Zed, even Gp, Rumble and many others belong in low skill floor high skill celling group.
Asol, Azir, Kalista, Kata, Nida, new Ryze little bit less than old one, but he still belongs here and few others have high skill floor and high skill celling.
The fact is, that picking up Riven for the first time wasn't ever really extremely hard, but to get real value out of her combos, mechanics, knowladge of matchups etc. you need to still invest many games into her. That's how otps get to 80%+ wr.
She has clear counterplay, she is strong in right hands and unguided missile in hands of new Riven player.
I think she has same problem as Yas/Thresh/Lee, it is incredibly satisfying to master them, thus their pr.