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.