To me personally, the point is the fixed supply, and the lack of central bank interference is just part of the means to that. I would like to be able to park money somewhere without having it lose value, and possibly even gain in value.
Anyway, as you said, someone certainly could fork bitcoin to make a system where the money supply is expanded indefinitely. I don't think it would catch on, though... at least unless bitcoin has already become "accepted."
You can design bitcoin with a slowly increasing bounty each year, as long as the protocol is there, no central institution can control it.
(as a matter a fact, if the 50 BTC bounty were to continue indefinitely, the rate of money expansion would decrease over time)