You could and probably should just include the information about parties joining, leaving or changing keys in the blocks so that there is no ambiguity about the

You could and probably should just include the information about parties joining, leaving or changing keys in the blocks so that there is no ambiguity about the involved parties. The fundamental issue is really making sure that a new party is authentic and not the equivalent of a fake account of an existing party. You could come up with something like the process when verifying a domain for a certificate, creating a special DNS entry or serving some specific content at a specific address. This doesn't really work because nobody stops you from registering arbitrarily many domains, it is just meant as an example that the process does not necessarily require a central authority. But there is a hard problem here, who is a party and gets one vote? Could a bank just create ten shell companies and then obtain eleven votes? Bitcoin kind of sidesteps this problem by implicitly declaring you a separate party if you possess some mining hardware and weighs your voting power by the hash rate of your hardware. So you are definitely right, Bitcoin achieves a degree of distribution that is probably really hard to do without a trusted central authority that has the power to decide who is a separate party and who is not. If the number of parties is small you could maybe still do it in a distributed way with some manual work. If a new bank wants to join they broadcast a request, use something like the mentioned DNS record to prove that the request is genuine and then all existing parties have to manually decide whether to accept or reject the new party and cast a corresponding vote. But this obviously doesn't scale well.