If 75% of the users want something, of course they can do that. They uninstall Bitcoin, implement a new software with the new rules, and install it. This becomes the new "Bitcoin". That is called a hard fork. You can't prevent that because it literally means people just chose to use something else. They may even keep the name and call the new thing Bitcoin, for what it is worth.

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.

Not only is it possible, it might eventually be likely. The supply schedule assumes that transaction fees will be sufficient for security, but researchers have cast doubt on that assumption:

https://freedom-to-tinker.com/2016/10/21/bitcoin-is-unstable...

If some future generation of bitcoiners is faced with either increasing the reward or putting up with frequent large rollbacks, we can't really predict which they'll choose.