Examine the contradiction in your words.
It ok , if ethereum switches to PoS , but not bitcoin.
Why one and not the other?
I'm not contradicting myself. PoS was never on Bitcoin's roadmap and will not be forked in ever (the far majority is actually against it to make it even more clear that it's not going to happen). PoS has been on Ethereum's roadmap for years now, and they might actually get it to work out. Different roadmaps, different niches, different ecosystems. It's just how things are.
PoW is quite powerful as long as the incentives align and the miners are following the rules just the way they are. In Bitcoin it's more profitable to be a brave miner than to be a rogue one. There is always theoretical risk, but that's with anything in life. Nothing is bullet proof. Bitcoin throughout the years has proven to be on the right track with PoW while PoS networks haven't shown much of what they claimed to be or wanted to achieve.