We have precedence on what happens if nodes/miners try to attempt a hard fork, it's called "Bitcoin cash". You need majority to agree on the rules, nodes/miners/users/exchanges all need to agree on what's Bitcoin, that's the whole point of a decentralized currency.

If a block size change caused the split into Bitcoin cash, you can only imagine what a cap change on Bitcoin available will cause, 21M cap is a lot less controversial than block sizes.