Oftentimes we still need a base we've built, which are expensive to maintain, but we dont need the large capacity any longer. Let us be able to downgrade the size of the airbase at no cost... just time... to simulate the moving away of personnel and equipment. That'd keep a reduced base there, where we might still need one... but not at the size or cost to maintain it. Currently, we can only destroy our airbase... which would cause us to start over at size 20 and cost a fortune to rebuild to medium size. So Reduce Airbase would really be a nice feature.
I'd wager that in a non-home area when there are no other nation colonies within amphibious assault range, the airbases could be downgraded from whatever wartime size they once were. That'd really help the AI if they could do it.