If anyone changes it and you don't like the changes you always 100% have the option to stay on the current network and refuse to upgrade. You can run the entire thing for yourself on one node if you want, some even do with fast block times for testing.

If the changes are bad the more likely thing is that everyone stays with you and the "new bitcoin" goes the way of new coke or bcash.

My point is the blocksize wars already happened so we know what happens if someone important and powerful changes bitcoin in a negative way. Tick tock next block.