The amount of bitcoin in circulation is very much determined by policies - the software of the bitcoin clients and servers. It is quite possible that mining rewards will be altered in the future as the amount dries up.
It's quite interesting in an economic sense too because, unlike our USD2 analogy, in the Bitcoin world users actually keep all their money in both forks (so if you have $1000 in USD, you'd also have $1000 in USD2) yet disproportionately favor one currency to the point that e.g. Bitcoin Cash is worth $217 per coin, while BTC mainline is worth $26,000 per coin.
The forks were the result of a split in the community. Not every change actually results in a fork.
If someone alters the amount of bitcoins out there, my wallet software won't recognize those bitcoins as legitimate.
What it is, as a project, is being extremely conservative, but that's a human political value, not a property of a technical system.