The difficulty does and has (in the short term) decreased when the amount of hashpower being used goes down.

Presumably if the price went down by a substantial chunk and stayed down for a while, the hashpower would also decrease, and so the difficulty would also decrease.

Also, if electricity prices went up, or if CO2 emissions were taxed, then hashpower would decrease, and the difficulty would go down in turn.

Bitcoin difficulty does not always increase and needs not always increase.

As for the rest... so what? It uses a lot of electricity and there is some pollution---but a lot of bitcoin mining is done with hydro or geothermal (and will be nuclear if bitcoin continues to grow), so, so what about some pollution?

And precisely in places like these, such as Niagara Falls, you see coin mining displacing tangible goods production due to both needing cheap electricity and one being more profitable than the other. In this case the market is not thinking very clearly in long term priorities, nor is infinite development of hydro and geothermal possible. Actually, this is about realizing that we live on a finite planet with finite resources that a finite amount of humans can finitely exploit finite parts of before the whole thing goes catawumpus. POW and current cryptocurrency systems are thoughtlessly and needlessly wasteful and represent inelegant architecture and brute force hackery like lightnig to correct what ultimately isn’t scalable: blockchain ledgers and fast transaction speeds vs centralization and speed. Look at DNS, for example, and how slow that is, and it’s architecture amd full vs partial copies of ledgers and jeiraexhical canonical lookups etc.