Bitcoin is PoW, it is in the whitepaper. However, if everybody agrees, yes bitcoin may switch to PoS as well but I don't think it is going to happen.
It is certainly never going to happen, maybe in another lifetime or metaverse, not this one 
Some people say PoS is not as safe as PoW and we still don't know if this is right. Eth and cardano doing well so far. So maybe, the hesitations might be baseless.
Time will tell, PoS hasn't been fully implemented on ETH network. As for Cardano, no one is sure how things would be if the network had huge transaction volumes.The rest of the other PoS experiments dubbed Ethereum killers in the previous bull run like EOS, TRON are already leaving below expectations in terms of market value