The supply of capital-B Bitcoin is 21M, but at any time, anyone can create Bitcoin2, which is an exact clone of Bitcoin but with a fresh chain. Any Bitcoin2 token has exactly the same utility as a Bitcoin token in principle - the only difference is that, for now at least, lots of people are mining Bitcoin and no one is mining Bitcoin2. If there comes a day when a lot of people think there should be 42M bitcoins, instead of 21, what would stop them from just starting a second chain? Granted, these "a lot of people" are not a "central authority", but is there really a difference when it comes to the question of bitcoin's total supply? As long as those people have enough compute they're willing to put to work mining Bitcoin2, there's nothing stopping them from creating more tokens that are just as useful as original Bitcoins.

And, of course, they could freely choose to make Bitcoin2 have a trillion tokens instead of 21M, or whatever number they want.