The rules of Bitcoin are fixed in place, and no one party can adjust them in future. ETH has core developers who can make any changes they wish whenever they wish.
The rules of both BTC and ETH are enforced by miners. At the moment ETH miners are generally amenable to changes by the core dev team. But there is no reason why future BTC miners could not do the same.
In particular, some of the same groups mine both BTC and ETH so it is clear they are amenable to changes.
Secondly, it's unclear why your presumption that change is bad is correct. The change to proof-of-stake for ETH is clearly a good thing environmentally for example.
