If governments forced miners to use solar energy only, would Bitcoin become more sustainable and valuable, or would it reduce mining capacity drastically?
We would see the hashrate reduced, difficulty adjusted and mining would become more centralized to sunny countries. Meaning fewer miners keep the network secure. This could make a 51% attack easier.
As far as the environment is concerned, the sustainability would not improved when it comes to hardware waste but carbon emmissions would drastically go down, which is a good thing.