Changes to Bitcoin rules require consensus by all the participants (people running and using nodes). The only things a majority of miners can do are attempt double-spends of their own transactions, or temporary denial-of-service by excluding transactions from the chain of blocks they mine. Any other changes will fork them off the network, and then they'd be minting coins with no economy to sell them into.
Secondly, any blatant attack on the network could be countered by the participants forking in a change in the algorithm used to do the proof-of-work. This would bankrupt the attacker because they would be sat on enormous amounts of now useless mining hardware.
Before Bitcoin was created I would have said such a system was probably impossible...
I guess my point is that the world is a complicated place to try to understand.
Major miners wanted block size increase but most of the rest of the world thought segwit was enough at that time and miners had to fork it off without taking over the original.
Miners can't go against the will of the users, especially if exchanges aren't going to like their changes as they won't be able to cover their bills.
Or another fork happens and the "primary" fork decides to have different rules.
Not quite "ever ever ever". Totally up to human whims.