If you get a more efficient PSU, miners' energy bill will be lower, which means the hash rate will go up by the same amount because the profit is not affected by mining efficiency (transaction fees plus block reward) whereas the costs (hardware plus energy) just went down. It would do nothing towards improving the environmental situation.

The only way to get miners to stop buying hardware or stop spending energy on this, is to lower the reward, i.e. if we decrease the value of a Bitcoin (stop putting more money into it) or collectively decide that maybe PoW isn't such a good idea in the middle of a climate crisis and try to bundle (do fewer) transactions or even stop using this system altogether. We can also try increasing energy taxes to the equivalent of climate impact (something approaching the cost of atmospheric carbon capture, on the order of €1 per kg of CO2e[1], because that's what it takes to undo what they caused), but that would just push miners to use up the cheaper renewable energy and crowd out everyone else and also impact people that use energy as a modern life necessity rather than for fun and profit.

[1] https://www.climeworks.com/subscriptions