let's get the obvious out of the way first: Video game logic: everything's okay as long as it facilitates the story. Don't think about it so hard. Now, that said let's see if we can make it make sense. First, numbers: The impression we had prior to D2 was that all of humanity (more or less) lived in that one City. D2 doesn't exactly dispute this, other than confirming that there are people living outside the City walls. If all of humanity is confined to one city, our numbers total.... what? New York City alone has a population of over 8 million people and the planet has a population of 7 billion. We know the Collapse pushed humanity to the brink of extinction. My guess is humanity probably totals no more than 5-10 million. With numbers that small, we're don't have the manpower to retake the system. Further, human lifespan might be shorter again after the collapse, without all the golden age tech to extend it. Second, resources: We're still scrounging for golden age tech, which means we're not back to that level of development yet. Have you ever seen a large Vanguard ship? I've not, just the small single-man jumpships. Seems to indicate we don't have the tech or resources to build ships any bigger. My guess is that we know how to fix golden age FTL drives, but we don't actually know how to build new ones. Which leads me to... Third, knowledge: There is a lot about the golden age that we still don't know. So much information and history was lost in the collapse. It's very similar to how the world regressed after the fall of Rome. It took Centuries to get back to that level of civilization. I think the city is surviving on a myriad of patched-up golden age equipment, along with maybe some reverse-engineered variants thereof. But every time Golden Age relics are brought up there's an almost reverential attitude about them. Like present-day people can't believe such marvels were once commonplace. Golden-age humanity created and controlled several warminds: City-age humanity cant even talk to one. We have the foundations for rebuilding our golden age, especially now that the traveler is awake again. But we likely dont have the time. The fallen have been united and they're still fighting us for resources in our own back yard. The Cabal have been dealt a huge blow with the death of Gaul, but Calus is still out there and could in theory assume control of the Empire again. Savathun's coming with her Taken (the fall expansion, likely), and the Pyramid ships are coming from outside the galaxy. We had something like 400 or 500 years of Golden Age to build up our tech, but lost it all when the Darkness came. We likely dont have that kind of time now, and we're worse off than before.