You've added a restriction that it must be changeable by an individual.
You mean that you didn't read my question the first time right, asking if you can change it, obviously a master group of hackers with unlimited resources can change it, but it's of no consequence just as it's of no consequence that a consensus can happen and data can be removed.
The probability of it happening has no bearing on whether it is immutable. The ability to change it is baked into it's design. Just like ethereum is not immutable they changed the blockchain.