You're kinda missing the point of asteroid mining. The idea is to gather useful materials up there and keep them up there to support a space based civilization. That way you can use and move the stuff around using energy efficient transfer orbits and modest amounts of delta-vee, instead of lifting mountains of material against Earth's gravity. A lot of high-tech from Earth will be needed to bootstrap early operations, but the goal would be a self supporting space-based society based on raw resources available in space, eventually trading with Earth-based societies.
I don't know how long developing such a society would take, but it'll likely be started by people nearly everyone else considers lunatics. Something like Mars One should be a rehearsal for later attempts at colonizing space itself, without a planet under your feet.