If forges are limited like this, then I do not want limited lifespan at all! I would hate it!
That means eventually, all forges would be consumed and building pristine ships reliably would be impossible. Without forges, max quality from stability, orbital works, and doctrine would max at about 95 or so percent - not high enough. Also, after forges are gone, I would be forced to use AI cores to meet demand of some vital structures, especially for megaport and waystation. After Pather cells get fixed next release, they will probably be a major headache because they are immortal zombies like pirates. (Currently, bugged Pathers that do nothing serious are a good thing because that is one less babysitting headache to deal with. The Pather bug is a good bug!) I would also feel compelled to wipe Hegemony off the map by whatever means necessary to eliminate inspections.
How quickly are forges consumed? If it is a matter of years, then there should be none available because they would be consumed in the 200 years before the game start. If not, then there is no point because they will last long enough before the game ends. Maybe player can use up one or two for really long games. Probably no more than an annoyance - not worth the hassle.
Re planetary shieldIt is a special blueprint. After it is learned, player can build as many as he wants.
And since the Hegemony already seizes AI cores, I don't see why nanoforges would be considered legally traded tech. They are the most powerful tech in the sector. Should probably be treated like AI cores.
They seize cores not because they are rare and valuable. It is because the Domain outlawed high AI (Hegemony considers itself its successor, and is trying to restore it) and high AI is dangerous and potentially treacherous. Alpha AI is more human than human in Starsector.
Forges are just rare and highly valuable tech. Well, maybe not that valuable in terms of money, just a big ship or a colony structure. It is not a demon that can kill all humans if it feels like it and gets the chance.
For me, in vanilla there is only one reason for sector to die: pirate hordes destroying everything they see. I mean: hey, if player faction can copy such complicated tech like planetary shield, i dont see why cant scientists re-invent blueprints and forges by studying the samples.
Currently, lore like AI wars make no sense because pirates are stronger than core worlds (because factions do nothing to stop them) and will decivilize them after some years, less than 200.
Players who play until they build an Ordos smasher fleet and quit soon after, probably five or so years, not long enough for pirate raids to do much. For epic games where player wants to leisurely explore and colonize the sector, that takes decades and worlds decivilizing is a real possibility.