I'll give a slightly different perspective than the two existing (great) answers you've already been given, and answer to a comment you made:
Great, but the point is more or the less the same. The idea that 'there will only ever be 21m bitcoin' has the same technical integrity as 'the blocksize will only ever be 1MB'. Whether one is more likely to change or not, is a social issue, not a technical one.
Yes, technically speaking changing the block size is equally hard as changing the total supply or inflation schedule. They both require a hard fork, which means new software needs to be deployed by every network participant, or it will just be a separate currency that forks off.
However, the reason hard forks are hard is because of ecosystem expectations. If participants believe that certain changes are hard, they will be hard. If people believe it to be unlikely that a certain rule will change, they'll need enormous amounts of evidence that indeed every one thinks differently before they'll even consider upgrading to new software that implements this new rule - as they don't want to end up on a separate currency alone (this seems to have been what happened with the B2X fork). If people believe that some group effectively dictates the system's rules, they may just go along regardless (this seems to have been what happened with Ethereum Classic). In my opinion, the ecosystem's assumption that certain changes are hard is the very thing that makes Bitcoin valuable.
So, indeed, it is a social issue if you're talking about highly invasive system changes. But that does not mean every hard fork is equally hard. Ultimately, the system's users define what the system is (which includes miners, but certainly a majority of miners can't arbitrarily decide the rules if it's against the will of the system's users).