If governments forced miners to use solar energy only, would Bitcoin become more sustainable and valuable, or would it reduce mining capacity drastically?
Solar energy is not the only form of renewable energy that is available so the government cannot restrict miners to only one form of usage. They can actually suggest to this miners and make policies that encourage more renewable sources for the mining activities. Most miners will appreciate a friendly mining environment even if it means being restricted to having the privilege of choosing a renewable source of energy to use for their mining.