I don't believe that will ever happen. PoW is more secure and decentralized than PoS. In Bitcoin, miners and holders are two distinct sets of people. In PoS, your wallet balance has direct influence over network security. The distribution of Bitcoin held in different addresses is very top-heavy with whales dominating it. If Bitcoin whales controlled not only the token distribution but network security as well, I think a lot of people would lose confidence in the system. Besides, Bitcoin can scale through trustless off-chain second layer solutions.

Ethereum is a smart contract platform. Smart contracts are executed on the blockchain level, which means that second layer scaling solutions are off the table. Sharding, sidechains - or changing the consensus mechanism to PoS - are the only scaling solutions available. It makes much more sense to migrate Ethereum to the PoS consensus model.