Fine, let's talk about practicality. The practicality of a blockchain protocol which is never allowed to 'update' via a fork is fairly kludgy. So if we allow fo

Fine, let's talk about practicality. The practicality of a blockchain protocol which is never allowed to 'update' via a fork is fairly kludgy. So if we allow forks, how can we be sure that in practice, for users, their account balances are immutable? Well its easy, a sufficiently decentralized protocol, like Bitcoin for example, can have a large group of miners and nodes agree to scam one or more users out of their account balances by simply forking the chain. Anyone not in on the scam, even if it was as small as to take $10 from a single user, has no reason to even consider that fork - the fork which scams any user to any degree is worthless, as well as being provably invalid. It only takes one node with the honest history to maintain what users care about. Saying this is practically mutable is like saying today's newspaper is mutable because we can all agree to print an altered version of it, while ignoring the fact that it is trivial to find and prove ownership of the original (being generous with my analogy, carbon dating, time-stamping the paper, w.e.).