But the protocol IS the coin.
Well, no. In addition to the protocol, the coin is also the ledger.
Yes, you're right. I meant to say that if you fundamentally change the protocol, you have a different coin. But of course, if you start a new chain, you ALSO have a new coin, as you correctly point out.
I don't believe you are right on your immutability claim. Plate tectonics may be analogous. Two forces are working in opposition. As the stresses get higher and higher, eventually something snaps, and a new equilibrium is formed.
My idea is: where does immutability even come from ? Why should people join a system of which the rules can arbitrarily change ? Would you buy a coin of which chances are that tomorrow, "a majority" votes that you don't own that coin any more ? Would you trade value for transactions that can be undone the very next day "by majority" ?
Even though in principle, any consensus mechanism can change just anything, any time, you would EXPECT that the basic rules you subscribed to when entering the system, are "enforced" by some mechanism, no ?
But as that "mechanism" has no centralized leadership, and is to be trustless, it cannot have any "moral decision power". It cannot distinguish which changes are in fact "good", and which changes are "bad". So there *must be a mechanism that enforces stability of the rules*. That mechanism is what guarantees immutability. If you don't think that such a mechanism is at work, you must presume that there are only 2 possibilities:
1) there is a central form of authority, that will distinguish good from bad ; for instance, a genuine majority vote inspired by charismatic leadership IN WHICH YOU HAVE TO HAVE TRUST
2) anything can happen any moment, and your holdings, transactions, whatever, can and will change in an ARBITRARY way any moment.
In 1) you don't have a decentralized, trustless system. You have something very similar to normal banking.
In 2), you have a totally arbitrary system that is unstable. Impossible to generate any reasonable monetary belief.
My idea is that 1) is possible, but that there IS a mechanism at work: decentralized antagonism. No antagonist is strong enough, and can find enough collusion, to get HIS changes imposed by majority. No majority can be reached over anything else but status quo on IMPORTANT (economical) aspects of the system, simply because there are different ways to change the system and too many different antagonists which are not colluding and have no common leadership (= my definition of "decentralized").
As such, only status quo can have a majority "vote" by so many non-colluding antagonists. This is the cause of immutability, and the fundamental property that can make decentralized trustless systems develop monetary belief.
If there is a majority that imposes change, it means that the system was centralized under leadership.
However, you are right that hard forks (new coin creation) can be performed by just any antagonist. In that case, people vote in the market: but this is not "leadership". Any antagonist, no matter how small, can do this, and the success of his modification emerges in the market, leaderless. The old system, however, remains immutable. (but can die)