The materials harvested are transferable. So is the tech, it won't just be colony tech...
At this point a planetary colony and asteroid mining are both in the realm of achievable fantasy. We can get there but are not there.
As for NASA seeing Mars as a populist issue that gets them attention. Of course it is. It makes them relevant in the eyes of the public and brings in money and tech. That still makes Mars a focus.
A manned mission is not planned mid century. It is planed in the 2030s.
Is there even a time frame for asteroid mining? I did not see one after a brief google search.
Now you're being hard-headed for no reason. Materials harvested on Mars already exist in larger quantities much closer to Earth (hell, they exist in much larger quantities ON Earth). No new tech will come from a tiny *** colony on a different planet.
2030s is in the "mid-century" range, specially that delays are a guarantee in large scale space projects.
There is a time frame for asteroid mining. Currently, companies are deploying small orbital telescopes to map near earth asteroids and plan which ones would be easiest to mine first. The hope is that by mid-20s robots can be sent to mine them, and bring back the materials closer to Earth. The time-frame for establishing orbital mining stations is about the same as a manned mission to Mars, and a lot sooner than any prospects of trying to establish a Martian outpost. By the time the latter is feasible, we'll be able to construct sizable space habitats.