I think that you're ignoring the SUBSTANTIAL momentum change that's required to get a serious chunk of asteroid down to Earth orbit. THAT fuel would need to moved all the way from Earth to the asteroid orbit. Note that most asteroids are not rich in aluminum nor titanium, so prospecting for these materials will be none trivial and will cost a considerable amount of fuel just to do the prospecting. Then, we need to get rid of the smelting waste, so again, a truckload of fuel is required to deorbit the waste material. The minimum fuel trajectories take something like 10 times longer to do the trip, so we'd be talking about stretching the basic fabrication process out by decades, otherwise we'd need a massive amount of fuel.
Even though the ISS took nearly 20 years to design, it went through something like 4 major design iterations, but in each iteration, the precise design solution was ostensibly known. At this time, none of the critical design solutions are known, since we have no idea how to deal with the propulsion. How are we going to get the major infrastructure into orbit? Building the ISS required some serious machinery to do just the metal forming and shaping, ignoring the smelting, etc.
Until we have solid solution with dealing with moving crap around the solar system without getting fuel from Earth, we don't have a viable design concept. Until we have a approach to get the large fixtures and machinery needed to do the basic forming and shaping of structures, we don't have a viable design concept.
TTFN
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