Basically it is a few lines of code in the protocol an the fact that the majority of miners and users follow that protocol.
It is horribly trivial to change the supply of bitcoin to an arbitrary amount if there was enough backing by the community for it.
Saying it is impossible to change is having a fundamental misunderstanding of how or why bitcoin works
Bitcoin was meant to be an e-payment method (decentralized, trutless, etc.). A fixed bitcoin supply is not necessary for that goal. Indeed, bitcoin is being used in that role, in spite of still having 10%/year inflation (and even higher in the past). And dollars and euros work fine as payment methods, in spite of their "horrendous" 1-2%/year inflation rate.
Thus, the argument that "raising the emission limit would destroy the value of bitcoin" does not sound convincing. Hoarders would be very unhappy, of course. Miners, however, may someday find it advantageous, especially by the time they are expected do depend on transaction fees instead of block rewards. Block reward is steady and predictable, whereas fees depend on transaction volume -- which will probably shrink substantially if fees became mandatory. People who use bitcoin for payments may not care, or may prefer block rewards because they provides "free" transactions.
It has been argued that, if some miners tried to change the protocol, the rest of the network would stick to the old one. However, this correction mechanism has never been tested, and it seems difficult to predict what would happen, in all possible scenarios. (After all, it was "proved", with the same certainty, that altcoins would die as soon as they were born.) What if those "some miners" had 70% of the hash rate? What if a large subset of the users became convinced that the change was necessary for the health of the network, or got some immediate benefit from it (such as no-fee transactions)? What if payment processors and merchants accepted only the "new" bitcoin?
(By the way, some bitcoiners seem to be trying to convince people to adopt bitcoin by telling them that money sucks. I sense a problem with that marketing strategy: it seems that many people have used money sometime in their lives, and may even have enjoyed the experience -- unlikely as that may sound. )
All of this leads me to the conclusion that Ripple is the future of cryptocurrency. In fact, I am currently in the process of selling all of my assets to invest solely in XRP.
Jorge, thanks for your thoughtful comments. I'm bullish on Ripple too, but I wouldn't recommend anyone invest more than 50% of their net worth. Ripple is not a get-rich-quick scheme like bitcoin. It might take 6 months to a year before the first mega-bubble. Be smart and patient.