The laws of nature do, for instance.  Our knowledge of them, not.  Mathematical concepts do (in a Platonist perspective).  Our view on them, not.

Of course, bitcoin changed (ever so slightly) in the past, and maybe in the future, it will, but that simply means that bitcoin, as a decentralized trustless system, failed.  Note that bitcoin, until recently WAS centralized concerning the protocol: Core decided everything.  When Satoshi was around, bitcoin was centralized around him: he decided everything.  But with the advent of oligarchic mining and core trying to abuse of its power to counter them, core lost its centralized power, in the same way that the French king lost his head.  So bitcoin liberated itself from Core's central authority, but the price to pay is to accept the miner oligarchy (which, as @iamnotback outlined brilliantly, is kept in control by a few big whales).

If ever Segwit gets activated on bitcoin, we know that Core is still the central authority (like for instance, Vitalik is in the Ethereum universe).  BU will never get activated, because that was not its purpose ; its purpose was to kill Core's central power.

So, if we are still having these discussions next year, we know that bitcoin is decentralized and that an oligarchy of miners is playing its role of keeping bitcoin immutable ; if bitcoin activated segwit, we know that Core is the ultimate master and central power of bitcoin.  

I say next year, but in fact, the proof has been already delivered.  If core could have changed bitcoin, it would already have happened.  

So, yes, I agree that there's no guarantee that bitcoin will not change, but if it does, it simply proves that it is centralized.