If Kassadin is a late game champion, why is his winrate so mediocre late game?

For certain champions like Anivia, it can be argued that the late game stats aren't above average because they stall out lost games. Kassadin has very bad waveclear and is melee so this doesn't really apply. Thus why is it that on op.gg his winrate is 47% and 47th in the very late game?

Personally, I think his late game is merely average because a lot of his power is put into mobility and that makes his damage subpar. After more than 3 riftwalks, you rarely need extra mobility (esp. at that mana cost) so you're paying for something you barely use in terms of power budget. He can dive the backline carries and I get that it's powerful but unless he has Zhonya's, he just gets instaCCD and dies. Unlike Kat, Diana or Akali, he also rarely has sufficient damage to actually kill a carry in one rotation even if he's left alone. There are other champions with much better laning, more CC, poke, waveclear and still decent safety such as Viktor and they can frustrate the backline too. Viktor's ult. will kill a carry or zone them by itself late game. Why really pick Kass? He seems like a much less safe version of Ahri. Yeah, he has more theoretical mobility but he's way worse pre-6 and being melee itself makes laning less safe. The other reason I'm saying he's less safe is simply because he lacks Ahri's versatility. She can kite well but also poke and waveclear. She doesn't need to dive to do damage.

I'm not saying Kassadin is awful and I do find him decent into LB but it feels like he's definitely a bit undertuned. If his jump was as long as it used to be, I certainly wouldn't be claiming this. At the moment, I think he needs more damage scaling for late game or maybe a 10% CDR/lvl in his passive to make him worth it.