The entire point of Bitcoin is that you irreversibly give up something you value in exchange for creating a Bitcoin in return.
The Bitcoin now stands for that lost value, and there is now a corresponding psychological motive to replace the valuable thing that was lost (electricity, and all that goes with it including environmental impact) by mentally investing that loss in the Bitcoin token. So you protect it, you support the network that it exists within, you talk it up to your social circle, you do everything you can to make it as valuable to others as it is to you. All of these activities also take time and resources, aggravating the impact of the original loss commitment. You do this because someday you want to give it to someone else, whom you've convinced of its worth, in order for them give you back the value you lost by creating it.
There is no "environmental mitigation" for Bitcoin's energy use, this behavior (or a permutation of it in term of other forms of irreversibly burned value, like time* ) is an essential property of cryptocurrency, without which it would cease to exist.
Disclaimer I hold Bitcoin, not because I think it is a new form of money or "decentralized" in any way that matters (quite the opposite in fact), but because it is possibly the most clever and subtle ruse to access the gambling instinct in a demographic whose constituents commonly believe themselves to be "above" playing the lottery or falling for Ponzi schemes. Yes, the irony is not lost on me. Still, this is a huge untapped market.
* also a bit of a dodge as IIRC POS still requires keeping a machine running even if it's not dedicating all cycles to the staking algo. Anything that "recovers value" from the process of "burning value" defeats the entire purpose of burning value to create psychological motivation to mentally allocate it to the token it replaces.
Which, incidentally, is why you won't see much growth from cryptos that try to do something "useful" (like fold proteins) while also standing as a counter for burned value. If running the algo is more useful for A, because A is in a position to monetize the work outside the currency being created, than it is for B, who can't use the data or results from the work algo, why would B prefer to support that crypto over another whose work algo is equally useless for all?