If governments forced miners to use solar energy only, would Bitcoin become more sustainable and valuable, or would it reduce mining capacity drastically?
What the government tends to enforce is tax revenue to the nation by forcing illegal mining to quit operation or get a license to operate so the benefits gets to the nation, Preventing environmental hazards by ensuring that mining firms are located at places their activities do not pose a danger to the public.
The government would never enforce the use of solar energy, and for the record, every country has their own unique means of generating electricity which is not always solar.
Come to think of it, what of countries in high-altitude regions that do not receive enough sunlight like Ireland, Iceland and even UK, how can your proposed enforcing work over there?