I disagree with this - bitcoin is advertised as mathematically unbreakable (which it currently practically is, on a technical level, due to the cryptography) - but this level of bulletproof security is promoted as extended to all properties of the blockchain - which isn't, on a technical level, true.
I disagree that's how it's promoted. You could make this argument about any piece of software (change the code to make it insecure, now look at the security hole!).
If someone were to do this with Bitcoin, users can decide whether to use the old chain or the new chain. Unlike a centralized service, no individual (or even group of people) can prevent you from using the version of the network you and your peers want to participate in.
