If the govt changes monetary policy, your held dollars change definition; if bitcoin devs do anything my bitcoins remain as per the definition i agreed to. I can choose to move to that new currency or not.
You can stipulate in a contract that it may only be settled with paper notes printer before 1974. It would be silly, since it would make transacting difficult. But no more so than holding some deprecated shard of a cryptocurrency. A better store of value are real assets.