Besides the problem of lost coins it also solves a big risk of most miners quitting when the reward dies out (nobody really knows that transaction fees will be enough) and the network is going to be vulnerable to attacks.
What are we really loosing? The block reward will not introduce that much inflation, because the absolute value of bitcoin produced will be less and less when compared to the total amount of bitcoins out there, and a tiny inflation is much better than limited amount of coins ever causing deflation (because of wider and wider bitcoin adoption, which is inevitable), because that only stimulates hoarding!
No, BeeCee1 is right. The miners do decide. Even if everyone else would screw them and start a new currency, who would mine? Miners are the only ones that will ultimately make the choice.
Well just a visible majority of *miners* is needed. If things would require 100% of everyone to agree, nothing would ever get done and nothing would ever get changed )). When the majority changes, the rest will have to upgrade or stay behind with a worthless chain, even if they would change a rule that you call "fixed". (I mean it is only fixed because the majority likes it right now. There are no contracts or any guarantees of any kind.)