It's not immutable. Bitcoin is a technology under development.....
Alright. Technically it really isn't 100% impossible to change the supply limit, because it could be done through a hard fork(though it really wouldn't be BTC). But good luck trying to convince people to follow a chain with a higher total supply. Bitcoin being scarce and slowly dis-inflationary are very very important characteristics of bitcoin both as an investment and as a store-of-value.