> And yet more people use dollars, and have bank accounts with dollars or their home country's currency.

Those people are losing money due to inflation. This is well known and an issue with people keeping cash in "mattresses".

> Bitcoin's block reward can be changed as well, it's not some immutable law of the blockchain. I'm not claiming that it will change, but I also see the potential for there to be a change there to incentivize mining once the block rewards are minuscule and miners are dependent on transaction fees for profitability.

This is called a hard fork, and has been done more or less unsuccessfully multiple times now.