Doomed - doomed, I tell you
Which is why ultra-large (crews much larger than a dozen people, tops) crewed space vehicles will never happen. As the size and complexity increase a combinataorial explosion will eventually lead to a literal one (or perhaps a literal decompression or loss of attitude control or similar total-loss-of-vehicle-and-crew event. There are just too many things that can go wrong which lead to everyone dying. If you only have, say, 30 thrusters attached to the ISS and they only go fatally bang once in 1000 years the MTBF is probably acceptable. What about when the vehicle's so huge it needs 30,000 thrusters? (For thrusters, read any other component that kills everyone in certain failure modes.)
That's not an argument against crewed flight, btw, it's an argument against idea of vehicles or facilities with more than a certain threshold level of complexity, There are other arguments against having any crew at all :)
Aliens because maybe they have magic fail-oroof technology.