How easy / hard would it be to change this limit? It seems,
this function controls how many coins are given to miners for solving a block. If developers decided to change this function, the 21 million number would change too, right? Or is the 21 million number somehow coded deeper in the system?
If there is a block containing larger-than-standard reward, it is rejected by standard nodes. If there are some non-standard nodes who accept it, then they effectively create a blockchain fork, and onwards they are on their own.
Or a different point of view, it would take a hard fork with all it takes.