Figure you could have both the Reavers pushing farther out every year, and them having a finite limit before they can’t do that anymore. Eventually, they’ll hit a point where they can’t push out further and they’ll start to collapse inward from lack of resources and personnel, but they haven’t hit that point yet (well, maybe they did after the shootup between the Reavers and the Alliance fleet at Mr. Universe’s hideout, where it looked like the Alliance guys had the upper hand once they got over the initial shock)
Also, standing your ground and shooting the Reavers up sounds like a solid plan, until you consider that the majority of people flying around are probably minimally armed for self defense at best. Serenity’s crew, being outlaws generally, are better armed than you’d expect many people on a transport to be, and even they would rather play possum than pick a fight with a Reaver ship, and indeed, the only times they ever fight with the Reavers (all during Serenity), it’s either because they have nowhere to run (Mr. Universe’s hideout), have no choice but to fight as they run (the bank heist in the beginning, running at this point only having a real chance of success because the Reavers are busy ravaging the town), or because they want the Reavers to give chase (Miranda, on their way to an Alliance ambush at Mr. U’s hideout)
Other than that, the only guys shown to successfully fight off the Reavers are River and the Alliance forces themselves, more heavily armed than even Mal’s crew could hope to be. It stands to reason that anyone who stood their ground and fought didn’t get to shoot up a significant number of Reavers, or even worse, given that many folks debate the existance of Reavers, decided to run instead when confronted and were simply run down rather than putting a fight up at all.