This might strike some of y'all as too political for the moment, but today I finished what I'm pretty sure is the longest economic boom event I've ever seen--just shy of 50 turns. I don't remember any booms much near half that long.
GW, it may be a bit too much cheese for your liking, but i have had econ booms last over 5 years. Set your autosave to one turn and if you lose the econ boom, just reload the previous auto save and keep going. The econ boom will still continue.
Just rinse and repeat. I have had to do it say about 10~15 times in a 6 or 7 year game, but retaing that boom is often too much to pass up.
@OP. I do beleieve there is also a treasury penalty at 10,000BC as well. Though i am not 100% sure on that but as i recall it is no where near as severe as the 20K one. Having a gentle penalty at 10K though does make the 20K a bit more understandable in it's severity.
But your certainly not doomed if you continue to incurr the penalty. You are still making BC, albiet at the reduced penalized rate but you can continue to prosper. I have had millions in the treasury in games before, i just simply ignored the penalty.
Where it does hurt you though is in your econ score. Your econ score is based on pure income before all leases and penalties. So in comparison to a under 20K treasury incurring the penalty will hurt you in the long run score wise. If you don't play on the MV though, then this is likely to be not worrisome.
As for modding it out, i seem to recall discussion on this in the past, and IIRC i think it is hard coded. Someone like Zyxplosion or a Dev though would be better equipped to answer that for you 100%