Bitcoin is NOT mathematically limited to twenty one million bitcoins.
I do not support raising the cap at this time but if we had overwhelming support like we did for Segwit we could raise the limit to any number we could dream up.
I would have no issue changing the code to allow a 1 bitcoin reward for every block forever... to ensure a strong distributed mining community.
I understand this must sound like blasphemy to the majority of the community but it irks me to see such a false statement about bitcoin repeated over and over again in every Bitcoin youtube video out there; there has to be a better way to phase it.
With the same reasoning that someone can fork to implement any changes (and some percentage of community might accept it as Bitcoin) you could argue that Bitcoin is not decentralized, not trustless, not immutable, not irreversible, et cetera, et cetera. This doesn't really proves or means anything, because consensus structures can't be inherently unchangeable. One could argue that all Constitutions are useless, because majority could vote and change them however they like.