This is true, they could hard-fork the chain and make that the new Bitcoin but this is where a bit of Game Theory comes in - why would the network collectively do something to harm it's own self? Bitcoin miners are incentivized to keep up the security AND stick to the 21 million limit laid down by the Bitcoin Whitepaper. If hypothetically the network decides to change the limit, I believe that would just destroy the Bitcoin narrative and so no miner would even agree to do that.