It's not that it's fixed. It's that if anyone tried to change it, the choice is upon the user what to enforce at their node. We aren't guaranteed that everyone will continue to enforce that fixed monetary policy, but I know I will. We know that anyone who has Bitcoin to transact with, such that they'd need a node to do it, is incentivized to maintain that policy.

He's not wrong, it can change. We have changed it. Bitcoin remains and also the change happened. The forks of Bitcoin run a changed monetary policy.

If Bitcoin started with 2% inflation and a viable fork changed that to 0%, it's still Bitcoin, but so is the original. Viable in remaining decentralized and trustless and secure (no big blocks, large and distributed community, hash rate to avoid 51% attacks).

In my mind, because of the incentives at the individual hodler level, users would go for the 0% one. If there were no other changes to that fork, it wins mathematically. Changing something doesn't make the whole concept of Bitcoin break down. You still have the power to choose to change or not, because blockchains don't force you to change. You get both ways, and the best choice wins.