So turrets would rust and acid tubes would rot.
The need for maintenance might be an interesting idea on its own. That turrets lose a bit health when they deal damage.
But back to the topic.
@Marietto so your suggestion would be to abandon the miners and have the same principle of eggs and repeaters from tremulous again.
With a maximal number of outpost with each having there own fixed pool of available build points.
I think there was once a tremulous mod with exactly this principle, but i cant remember if it was good or bad.
At the first glance that system seems not better or worse than miners just a different approach. Being able to build freely feels nice and makes it easier to use than an imaginary sphere around an outpost. On the other hand placing miners is more or less the same problem to visualize. Hunting miners is interesting but provides not much reward.