I would love if infrastructure protected population from negative population growth, particularly or spacifically underground infrastructure, I'm thinking of using it basically as fallout shelters for keeping some of your population alive in the event of nuclear war, the speed at which a planet's population is reduced to nothing after a few missiles has always unsettled me, fallout from modern clean nuclear weapons isn't particularly bad even when it's global extinction level nuclear war. Obviously food/clean water shortage, disease, civil unrest etc would be what finishes everybody off, but It would be nice to be able to provide some insulation against this happening.
Also related to this perhaps the underground infrastructure could also protect a small amount of industry from destruction, perhaps some destroyed industry could be converted into conventional and then be mostly cushioned against destruction, continuous bombardment should eventually destroy the protected industry, population and the underground infrastructure itself, but it should be much harder to remove the last vestiges of civilization.