No upkeep would mean no staff which would mean your base is shutdown when you aren't there, which would only please some of the people who don't want base building.
Why must it keep running while you're away? It's a static base, not a place for trade and such.
You could have somewhere for other players to refuel, repair and rearm if you wish, but that's about it. Even they could be freebies with an automated system that doesn't need filling up since the base wouldn't make you money, it's just a place to store your stuff and maybe have a storage facility with a refinery for your mining purposes, but you and some drones operate all that. The 4 modules are simply a way to make it more difficult to complete rather than for it to be functional as a base with staff. Drones don't need wages.
The very occasional player that chances across it can get what they need gratis and be on their way. Having a trading hub in it would be meaningless since, it's so far off the grid that nobody is going to travel that far just for trade.
Quite honestly, if they make them require upkeep like the FC, it would be a complete disaster.
They will either have to be close enough to the bubble to make it viable to keep hopping back and forth to earn the upkeep, which makes it quite pointless since you already have stations and FC's to do that all over the place in the bubble.
The other alternative is to give it a way to earn money for itself. That won't work unless it's also close enough to the bubble for other players to visit it with trade.
In effect, you would kill it completely since some players would probably want to go completely off the grid, halfway across the galaxy. If it always required you to be close to the bubble, there's no point in doing it at all, the number of players would never spread out far enough to make it worthwhile. The whole idea of having a base for yourself would mean that explorers could be halfway across the galaxy and chance across a players base, a slim chance maybe, but it's still a chance. If they are marked on the galactic map if they have facilities for repair, explorers could get repairs done for the odd mishap that always happens at some point.
Why do people live miles away from anywhere and have their nearest neighbours 100 miles away or more? Why don't they all live in big towns?
The answer is: Some people like a bit of peace and quiet where they can relax and enjoy themselves. Call them eccentric or unsociable or whatever you like, it's just human nature.