So I would like to add a possibility: if the necessary energy to move the mining activity was supplied *for free*, could btc transactions be executed for free as well?
it will always require capital to acquire solar panels and other renewable energy infrastructure. it doesn't come free. just like now, miners need to invest in hardware and infrastructure to indirectly invest in BTC.
as bitcoin mining transitions towards green energy sources, it'll be better for the earth, but the fundamental incentive design won't change. we still need to pay miners to include our tx in blocks.