I didn't know that. I seem to remember I had above 50 k bc reserves in my last game But have to admit, I'm still on normal.
it's not an absolutely max; when your treasury hits 20k, your economy takes a penalty. i agree that it's annoying; it should at least be scaled to map size/ total number of planet tiles/ something.
but it doesn't kill me. a lot of people hate financing their purchases, but i worked in a mortgage company for a year and developed a think skin (it's all about cash flow!). for major upgrades, i will sometimes allow my treasure to shoot into -100k, but only if my gross income will allow me to recover in a few turns (12 is my usual max). it's a lot faster than trying to save up. sometimes i'll use a lease option with payments to help soften the blow. i've actually modded in the InterestRates tag as a point-buy race ability for my own use, which does seem to work.
i don't like the idea of requiring ships to be docked or in orbit. you can only have 10 ships in orbit at once, and the tedium of moving dozens of small fighters on and off a planet would suck! i'd have a newer ship design by the time i was done upgrading them all! it doesn't bother me that i can't see who's performing the upgrades - i can't see the cruise ships that're bringing me tourism revenue, either. i do think there should be times that it's impossible to upgrade, though. maybe a better middle ground would be that it takes exponentially more time and money to upgrade ships that are outside an SB's AoE (or the same distance from a planet), like this:
inside SB/planet AoE: 1 turn, 100% cost
1 radius outside an SB: 4 turns, 200% cost
2 radii outside: 9 turns, 300% cost
4 radii: 16 turns, 400% cost
etc.
and at some point it needs to become impossible. i've seen the AI with ships that're going to take 200+ turns to upgrade. these are usually gifted ships where the AI player's range does not reach - perhaps that'd be the best cutoff point; if the ship is outside the range of a ship w/o any life support modules, it simply cannot be upgraded.