Bitcoin is decentralized and every node owner has full power over whatever version they want to run themselves.
They could even run several and partake in several networks, e.g. a tentative fork to see how the fork works out.
It's up to each node owner to do what's right. Of course bitcoin core has an influential thought leadership there, so even if not formally in power a lot of nodes will follow their advice.
the "heaviest block wins" and "block reward can be some maximum" rules combine with network latency and block propagation patterns to ensure that a lot of the time, blocks that have the most common reward maximum, or less, have the greatest amount of time to be found compared to a fork with more
even nodes that have forked to allow bigger rewards than the canonical schedule will allow blocks with a lower reward
all that is required for them to be the "best block" is having a smaller block hash
so long as the majority of nodes stick to the schedule the miners have to stick to the schedule, yes, even the little humble node runners
because all it takes is a few miners using that old schedule as well, and 50/50 chance most of the time their block is going to be "heavier" and because of restricted propagation there is plenty of time for such side chains to become the winner in cumulative work (smallest sum of all blocks in the chain)
it is a 50% minimum divergence to break the security of nakamoto consensus, the "heaviest chain wins" rule
even if a fork with bigger rewards gets ahead by one block chances are that two blocks will be mined that are from the canonical reward maximum and one of those is smaller hash
unless someone also somehow gets 50% of hashpower to not just allow bigger rewards but REQUIRE them haha yeah, nah, not gonna happen
because of the culture of bitcoin, the chances of enough miners AND node runners updating to a new schedule are basically not much better than the chances of cracking a bitcoin key
