What you're saying is absolutely valid, and I think that's the big problem with that particular movie—because many people were expecting the typical zombie movie with a sequel, but the story turned out to be different, and this one is even more so.

These are stories that take place in a world that was destroyed by a pandemic of zombies or infected people—and that's it. These creatures are relevant because they're the root cause, but now they're just a side component.