In my mind, you have to separate the two different player scenarios - those playing actively, and those concerned with time between breaks.
a) if you play actively, by almost any definition of 'active' that you can imagine - 1 day / week or every day - the upkeep costs are math pov essentially negligible to zero factor. This isn't a comment about how and what you should 'feel' about the upkeep, just the cold hard math of any bizarre definition of 'active' play will allow a player to maintain their FC quite easily.
b) therefore, the real issue and/or pain point - is both the actual financial math and the 'feeling' pov of what happens if and when you take breaks from the game. Sure, there is decommissioning, but it is legit point to not want to run down your bank while offline. And sure you can manually sell, like I did when taking break from game, but the bottom line is --->
1) Fdev basically punishes you for taking break from game unless you are careful enough and take specific steps to sell your carrier first. This also discourages - even if only a tiny amount - players from returning to game. e.g. can't and won't say it is big or small factor, but it's reasonable to infer -some- negative obstacle to return is setup if the player feels like 'eh, I'd have to re-setup my FC, redock all my ships there, etc
For many players, this obstacle will mean almost nothing. For others, it will be the tiny extra speed bump that makes them click on another game in their library.
Bottom line point is I think regardless how you 'feel', the cold hard math is if playing even a few hours actively per week, upkeep is zero issue. So the real concern is the taking break from game point of view.
My personal preference would be that upkeep is simply halted when offline after XX days/weeks/months, whatever reasonable long term 'break' time estimate. Have the FC decommissioned in sense of removing from galmap, etc, but allow player to still own the FC, all ships docked stay docked, etc, and simply reactivate the upkeep when returning. You could also even add a similar penalty to like how rebuy insurance works - you pay portion of your earnings to additonal upkeep for some X duration of time based on your break length.
Because you are now back actively playing, paying an additional 10, 20, whatever percent more upkeep for some X weeks will be no issue because active play affordability has never been the problem. That's my .02 cents any rate.