the only practically immutable thing about bitcoin is that there will always be SOME market chosen canonical chain of blocks and a UTXO set deriving from it

EVERYTHING ELSE can and will change if a new consensus demands it: op-codes, block size, block rewards, hashing algo

and that's a good thing. because the property rights are in the UTXOs, and splits, forks, attacks are an evolutionary process of discovering the right set of trade-offs