Proof of Stake works in a different manner than Proof of Work. Yes it is a much efficient energy-wise and a much faster system for large-scale cryptocurrencies like bitcoin, but it would also require the consensus of multiple computation systems that will be unified under one single server. If everyone would be willing to chip in some of their computational power and unify it through the cloud, then proof of stake would work. Proof Of Work eliminates the middle man by allocating a dedicated computational system where transactions will be facilitated, this is of course only applicable for small forms of cryptocurrency with little potential for scalability.
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. 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.
Bitcoin too is doing fine with PoW, it ensures security and robustness of BTC blockchain. The issue of too much power consumption by bitcoin is as old as hills and IMO its just over hyped. Eth has not switched over to PoS yet as,
Altair, Ethereum 2.0’s first hard fork, is expected to come into effect by the end of the month — the point where the original proof-of-work Ethereum blockchain will transition to Ethereum 2.0’s proof-of-stake consensus mechanism. SourceWhether its PoW or PoS, they both are same way of climbing the same mountain. As PoW is centralized along miners that can afford expensive mining equipment and electricity bills, in PoS those who can stake huge amount of coins will grab the major share of mining.
When the last bitcoin is mined/bought, it would be harder for everyone to facilitate transactions more so afford them because the system will be overloaded and as a countermeasure, will charge higher fees and would slow the process down for the rest in queue. The thing about POS is it ensures efficiency of service and the price of each transaction which in theory should work, especially for coins with high potentials for scalability like bitcoin and ethereum, with these two being the most common victims of the shortcomings POW has.