[aeorgnoieang]

Those drawbacks are only true on some (possible) timescales. The biggest drawback currently, for human settlement of the asteroid belt, is that we can't afford to send anyone there!

Freeman Dyson's the person after which the Dyson sphere is named, so the current (relative) "unusability" of asteroids or the fact that humans currently seem to "do really poorly in microgravity" are 'just' additional obstacles to be overcome sometime in the future from the perspective in which space megastructures are possible.