I really should know better than to think logically about a zombie show based on a comic book...err...graphic novel;
But, that being said, where the fark are all the walkers coming from? They don't independently reproduce. Ricks group of a dozen or so has, by themselves, killed hundreds while losing a fraction of that. The one guy from the pilot episode has likely killed hundreds as well. You can reasonably infer Woodbury residents have killed 1000s.
So long as the kill to conversion rate remains that absurdly high, the walkers would disappear relatively quickly.