There are some downsides but also huge upsides aside from not wasting energy, for example offering very fast finality. Bitcoin has not moved to it, because arg

There are some downsides but also huge upsides aside from not wasting energy, for example offering very fast finality. Bitcoin has not moved to it, because arguably the new system would not be Bitcoin any more. It's a coordination problem where you have to get most users (including centralized exchanges) to stop following the PoW chain and start respecting the PoS chain, but they will only do that if they believe everybody else will. Ethereum was able to pull that off because Ethereum foundation and Vitalik (its creator) announcing and implementing, and practicing the switch many times, has a lot of weight. (And then still, some miners remained, but that chain is now known by a different name and not generally known as "Ethereum".) Bitcoin is a lot less coordinated in that sense, even less invasive changes to the protocol are difficult to pull off.