The key difference, though, is that fiat currency is easily debased because whoever controls it can print more, while Bitcoin does not have a single owner and can only change if the majority of miners agree to do so.
I’d be interested to know how many people would actually need to agree a fork that, say, didn’t halve the coinbase.
Also, of those, how many would give priority to the philosophical underpinnings of Bitcoin over making more money in the short/medium term.
Some will stop using it, sure. And they probably have enough coin to affect the price short term.
But I’d argue that the majority of usage is now not for political/philosophical reasons but for economic ones.
The longest blockchain which follows the rules. If a chain does not follow the rules, it will be ignored by the nodes, even if it's the longest one. To change the rules, you have to update all nodes, not just the miners.
The interesting part of the Bitcoin experiment is that so far it has had a very large inflation. This has been decreasing, and the critical part of the experiment will determine if a deflationary currency will work or not. My guess is that it might be in danger of collapse as miners will not want to mine it due to lack of incentive, meanwhile users would not want to transact since most will be hoarding it. Which is when the issuance rate might need to be fixed just to keep the miners rewards going...at least, it's one of the scenarios to consider...