BTC can't go PoS ever, the founder is no where to be found, if all PoW projects goes to PoS algo BTC will be the only one that will forver be a PoW coin, you need to do more research on BTC to understand that its unchangeable to another algorithm.
Bitcoin will forever be in PoW, Bitcoin is different from Ethereum, Bitcoin was the first cryptocurrency and it was designed for that,
but Ethereum is a cryptocurrency platform, so their uses are different, it's just that they are cryptocurrencies,
and keep in mind there are many more mineable coins for example Monero, ETC, LTC, CFX, CKB etc, don't worry if Ethereum moves in PoS.