If you look at the ETHBTC price chart there's a clear downtrend starting from the POS switch, so one would tend to connect the correlation with causation. Meanwhile a lot of the "crypto" talking heads explain it principally by ETH having its lunch eaten by Solana (and maybe L2s, maybe other chains). One could also connect the two, though: once you switch to POS you are competing only on functionality, not security; and, let's say, the former doesn't have scarcity (whereas the energy budget for security *is* scarce; this was a conclusion I came to all the way back in 2013 personally, when the first altcoins started to really try to compete. Bitcoin doesn't compete on functionality, it competes on scarce energy resources, at least at the deepest level.)

That said, there is still the perfectly plausible argument that a POS chain just isn't really "correct" (the tendency to cartelize, the lack of *robust* decentralization, perhaps). that's the whole argument about whether POS does actually replace PoW or not. I think not, but it's not exactly trivial to prove it.