This is a misunderstanding. Proof-of-work blocks do not act as clock for proof-of-stake blocks. Proof-of-stake blocks have their own difficulty and will adjust toward target spacing of 10 minutes all by their own.
Is the PoW clock just ignored then for blockchain validity purposes? If so, good idea. That means you can toss PoW entirely.
What do you do about PoS miners who report blockchains from the future? A lot of coin-age can be destroyed if we allow 2025 to reported as occurring tomorrow, even if only a tiny % of coins did the mining.