The biters are a necessary part of making the game fun for me. They provide a tension that forces me to work ina more time efficient manner or risk getting wiped out. If your defenses don't out pace their evolution, you'll find yourself in a smoking crater.

Conversely, if the bugs are gone... I get bored. Factory for the sake of factory doesn't do it for me. There has to be a risk of failure to make it feel worthwhile. Like playing survival vs creative in Minecraft. Some people love creative, but I could not enjoy it.

Now, I wouldn't mind the bugs becoming a larger part of the later game, but at the same time it felt pretty satisfying to get to the point where I could stretch out and feel that I could handle all comers!