"once we move to proof of stake" -> Alas, there is no PoS algorithm (who's security does not depend on PoW blockchain), so it's not "once" but more "if we ever move to PoS after we manage to invent it".
Polkadot is not "pure PoS" as well, but feel free to bring a citation to prove me wrong.
What does this mean?
You explained how this applies to ETH 2.0, how about Cardano and Polkadot?