My motivation is actually to have fun, maxing out weapons is just part of that. That might be the motivation for the hardcore, but there are millions of casual players that never even prestige, and just like to shoot shit.
By the time my interest wanes, and it'll be awhile, DLC will be here. Then Bad Company. By that time, I've probably sunk 100 hours into the game, and I can't really ask for much more from my $60 (in conjunction with single player and Spec Ops).