If governments forced miners to use solar energy only, would Bitcoin become more sustainable and valuable, or would it reduce mining capacity drastically?
Governments never agree into anything. There's never a world wide consensus.
This being said, some could maybe restrict to solar energy,
while some others
, would leave it wide open to earn more taxes off mining (i.e. hoping that many mining businesses will relocate there).
But this would happen if the governments would care more than just milking some taxes. Which is not the case.