> My next question is are there downsides to proof-of-stake, and why has bitcoin not moved towards it?

There are minor downsides in theory. Proof-of-stake is a bit less "democratic" and gives more voting share to those who have more money, vs. just are mining. In practice, proof-of-stake coins seem to be doing fine. The main one here is Ethereum.

The main reason Bitcoin hasn't is due to inertia and the interest for miners (who do proof-of-work) to use their democratic power to keep bitcoin invested in proof-of-work.