Does anyone have a good explanation as to why this is supposed to be hard to achieve? What incentive do miners in the distant future have against inflating a cryptocurrency?

For example, take Bitcoin after the cap has been reached and mining bitcoins only gives fees. If 50% of miners cooperate to make blocks worth real bitcoins again, they will suddenly start raking in more profit. They will be devaluing everyone's stores of bitcoins to do it, but people who hold bitcoins have no say in it, only computing power does. If we assume that miners will be the main owners of bitcoins for all eternity then they will probably never agree to this, but if there are a lot of miners with very little in reserve then why wouldn't they accept devaluing bitcoins a little bit to get more per block mined?

Exchanges and other prominent users would probably also have to agree to the change. Things might get bad if there was a split between miners and everyone else because the miners could perform 51% attacks on the other fork, though.