Forcing a hard fork in order to alter some aspect of the network doesn't even mean that you'll succeed in altering your desired aspect, but the resources required to fight the good hard fork fight are quite high, and completely unrecoverable. Yet, if a minority of users and miners decide to make that fight, there will be a blockchain fork, and the majority of the losses will fall upon the minority group until they can either muster up the resources to dominate the network or quit the fight and take their balls and go home.
It's not necessary to be a fight, two forks can co-exist since they serve the different interest from different users, its just those pre-fork coin can be spent on both fork, and it benefit the early adopters again (Pre-fork coins will worth more, people will try to get as much coin as possible before a fork happened
)It's not necessary, but both chains can't coexist unless one changes certain network parameters, such as port number.
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So, theoretically, all that is needed to change the 21 M BTC limit is to get everyone to agree to that change; but that isn't something that is up for debate here. No matter who or why, there is no way that a consensus to change that metric in Bitcoin is going to happen without a blockchain fork; and it would be a harsh fight. That number isn't so arbitrary, unlike the max_blocksize rule.
It does not matter it is MAX_BLOCK_SIZE or nSubsidy, as long as they can be changed through a software upgrade, there is a POSSIBILITY that a future software upgrade will change some bitcoin characters. This possibility is the biggest risk and uncertainty of bitcoin
I concede that the possibility exists. That's different than claiming that it's a non-trivial risk.