The difference is this would require a hard fork, and the value of this fork would be determined entirely by market forces. Imagine if when the US decided to 'print' a few trillion bucks, it resulted in the creation of a new USD2 currency. And you were free to keep using USD1 or swap over to USD2. More or less the exact same thing has already happened with Bitcoin multiple times, with the outcome you could expect.

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.

Not necessarily. The main BTC software has already changed a few times over the years, and nothing prevents it from changing again. If the BTC maintainers and major miners would agree, they could very well push this change. The same way they decided to stop expanding the block size, for example.

The forks were the result of a split in the community. Not every change actually results in a fork.

And the software of the bitcoin clients is at least in theory determined by the users.

If someone alters the amount of bitcoins out there, my wallet software won't recognize those bitcoins as legitimate.

The value of your wallet lies not in what you believe in, but what everybody else believes in.

The value of bitcoin lies in its immutability. That's the only thing it really has left going for it.

Bitcoin is not immutable. The current chain would make absolutely no sense to a (full-validating) client from 2011 due to various additions and protocol changes.

What it is, as a project, is being extremely conservative, but that's a human political value, not a property of a technical system.

I'm referring to the immutability of issuance and balances. That's the type of immutability that matters.