[...] Other than that, though, I think the story can be propped up reasonably well with some work.
My hypothesis: the Collapse is principally an economic and social phenomenon, and any technological regression is mostly if not entirely a side effect of these changes.
Picture the sector, pre-Collapse. Everything is made by autofactories, simply because they're so much easier/cheaper/faster/less labour-intensive than older methods. Very few systems actually have their own autofac, of course, but that's why the shipping industry exists. Presumably the Sector doesn't have the capacity to make its own autofacs or UACs.
Few of the colonies have self-sufficiency in terms of food production, relying primarily on imports. Many colonies are on hostile environments that require working high-tech equipment to keep alive. The Sector is a pretty safe place; if piracy or rebellion breaks out the Domain hears about it (eventually) and sends a fleet to step on it hard.
Then, the gates go silent. After a while, everybody realizes the Domain central government and the order and stability it brings are gone, as far as the Sector is concerned.
Suddenly, all the autofactories (the ones that don't get destroyed in the fighting, at any rate) are in the hands of any group with the guns to take and hold them. Trade and the economy crash, as foodstuffs and and key resources are raided by pirates or simply hoarded. Since nobody wants to sell (let alone give away) anything they can use if they can help it, when they have no idea where they'll be able to get replacements, the vast majority of people who were previously reliant on imports of such items will have a lot of trouble getting them and very little to buy them with. Any colony that doesn't have autarky and can't secure a trade route to someone able and willing to supply what they don't have is likely doomed, sooner or later. Things only get worse as time passes and equipment breaks down, faster if the technical staff who can maintain such things are all dead.
This is also why refugee lifts simply aren't going to happen on a particularly large scale - assuming they don't get seized by pirates or slavers on the way, the people fortunate enough to already be living on habitable planets are going to be quite reluctant to just open their doors. And even where such migrations succeed, the abandoned colony is, well, abandoned. As is consistent with the lore.
As xenoargh highlights: Why don't the colonies try to revert to previous-generation farming and manufacturing methods to support themselves, you ask? Well, they most likely
do. But it's not enough, for several reasons:
- Most colonies won't be able to build up sufficient replacement food supplies and economic capacity in time before things like famine and civil war cause their societies to collapse. If all the physical implementations of Green Revolution such as nitrogen fertilizers, widespread mechanization, and the factories that produce these things disappeared tomorrow, even if we lost none of the knowledge involved, the world as we know it would crash - violently - and wouldn't recover for a long time indeed. This isn't necessarily going to cause the system to "go dark" - not even the Black Death could put Europe down permanently - but it's not going to help them deal with the following problems.
- Most colonies aren't going to be able to defend themselves from being sacked by raiders or annexed by warlords before they can even get back on their feet economically.
- Anyone who tries to build a homegrown fleet using de novo designs is going to have to deal with the fact that they're basically doing the equivalent of reconstructing military aviation from scratch, when competing powers can just ask the autofactory genie for a fully functional F-22. The vast majority of such efforts just aren't going to reach a level where they can compete with Domain designs before they lose everything they have.
So there you have it. You can't keep a civilization afloat when the chief emotions in the social atmosphere are greed and fear.
Is the Sector screwed, in the long run? Not necessarily; if they do have the ability to make new autofactories and designs for them given enough time, they'll stay afloat indefinitely. Even if Domain tech is somehow Lost Forever, it needn't be the end of the universe - notwithstanding the points I made above, if an established power with a secure position like the Hegemony or Tri-Tachyon commits to a program of pure research augmented by reverse-engineering the Domain technology they have, they can rebuild to the equivalent of the pre-Collapse state in the long run, probably even fairly quickly.
The question is: with enemies on all sides and a Sector to dominate, will they?