So there's this magical limit of ~21 million bitcoins that can theoretically exist at max. Many assumptions about the future of bitcoin are based on this number.
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?
It's theoretically possible, yes. It's also theoretically possible for the quantum force we call gravity to reverse itself tommorrow, and we are all 'reborn' in the result, that some scientist calls "The big fart" in another 14 trillion years.
I'd give those two about even odds.