As for the environmental impact: individual Bitcoin transactions don’t directly increase energy usage. Miners consume electricity regardless of whether any given person transacts. And while mining is energy intensive, it doesn't have an environmental impact in itself.
The context I was looking at related to storing data in the bitcoin ledger. I think I commented here, I'll have to look back and see.
When you send a transaction, you get to pick what fee you'd like to use. For this kind of service, transaction speed isn't really important so you can get away with setting a low fee.
Here's a 0.15$ fee transaction that was mined just 6 minutes ago: https://mempool.space/tx/2e66ba1758e233ff1b2395c7db55f8327de...
By "off-chain", I meant something like BuyMeACoffee letting users top up a wallet via an on-chain deposit, and then handling tips internally (e.g., in a SQL database). That way, small payments don't each require a blockchain transaction.
But in general, I agree that cryptocurrencies could help the problem, but it still carries the compliance and AML risks unless you can outsource that to a 3rd party. What happens if you accept funds from an OFAC or other sanctioned account? Violating AML can come with huge fines and is generally just a massive headache. Cash doesn't have these issues as the evidence is generally impossible to track well. With nearly all cryptocurrencies, the entire ledger is open, so if you accidentally accepted payment from the wrong person, you could get royally screwed at some later time.
The only thing that can change value is if the global hash rate significantly changes. This is why we have the difficulty readjustment every 2016 blocks (give or take 2 weeks).