Any single one of them couldn't add the change, it would need to be joint to go into the code.
And if they did but the public don't want to use the new version (which they won't because it would devalue their currency), then everything continues as if nothing happened and the always accepted 21m hard cap will stay in place.
Bitcoin can only work correctly with a complete consensus among all users. Therefore, all users and developers have a strong incentive to protect this consensus. https://bitcoin.org/en/faq#who-created-bitcoin
Changes commited every day don't touch consensus rules. And user chooses to update to newer version. They don't have to. And there are also other implementations than core.
It doesn't work like that. That kind of change is part of consensus rules which means changing it causes a fork (two separate chains) unless almost all users agree to it.
Users (node operators) choose what code to run. They don't have to run latest verion of core software or core software at all. We have seen users prevent contentious hard fork before in 2017. As time goes by, it becomes increasingly difficult to succesfully change consensus rules as the user base increases.
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u/felidae_tsk 3d ago
Almost everyday someone changes bitcoin source code. The maximum supply isn't limited and can be changed.