During a discussion in the Discord server, I realized that the two tech lines related to the maintenance facilities are not balanced in the following sense.
Each maintenance facility has a capacity of x tons and can produce y BP worth of MSP per year, where x and y are given by the 'Maintenance Support per Facility: x Tons' tech line and the 'Maintenance Production Rate y MSP' tech line. We also know that each MSP costs 0.25BP, and each ship requires 1/4 of its BP's MSP to maintain annually. Thus, if the ships being maintained have an average 'cost density' above (y/0.25*4) / (x/50) BP/HS, the MSP produced by the facilities will not be enough to maintain the ships. I.e., in this case, you need more maintenance facilities for their production capabilities, rather than the maintenance capacity.
The cost density (BP/HS) values for each 'tier' of the two tech looks like below:
The cost density thresholds look somewhat reasonable for low tech levels but are pretty low for high tech levels.
What is more surprising to me is that the BP/HS threshold goes
down as the tech lines progress. This is counter-intuitive as the ship components get more cost-dense as the relevant techs progress. Thus as the tech progresses, the maintenance capacity tech becomes less and less useful, as you need a lot of maintenance facilities to produce the MSP needed anyway.
I suggest the MSP production tech line to be slightly buffed, so the overall cost density threshold increases slowly as the tech progresses, which makes the other line not obsolete after certain tech levels, and reflects the overall cost density increase of the ships when tech level progresses.
Of course in the above calculation, I ignored the production bonuses from governors which buff the MSP production. Meanwhile, I also ignored the MSP needed for overhaul, for the MSP storage on ships, and the ship/station based maintenance modules which maintain ships but do not produce MSP. I think it is reasonable to assume these two cancel each other, if not further debuff the overall MSP production rate.
An additional note, the tech line 'Maintenance Production Rate y MSP' actually produces y BP worth of MSP per year (4y MSP per year), thus the name itself is ambiguous. It is more clear to name it as 'Maintenance Production Rate 4y MSP' or ''Maintenance Production Rate y BP'.