> The leadership can decide the rules of a new fork unless everyone else coordinate to oppose it?
You are 100% free to not install the latest upgrade on your node(s). Unless you have enough people who agree with you and coordinate their refusal to upgrade you will be a network of one that goes entirely unused. This is exactly how decentralization is supposed to work. You have to obtain some level of consensus to get anything done. This isn't a situation where "everyone" has too coordinate to oppose something, just "enough" people to form a fork that has enough hashpower/security to attract users. Even if a large enough minority of miners were able to coordinate a new chain, fork the code, make the necessary code changes and start their own version of Ethereum they still have to get people to use their network.