Asteroid mining would be so time, resource and finance consuming that is seems wasteful compared with terrestrial solutions to this resource crisis. Why go to the trouble of building this non-existent technology, and travelling unthinkable distances, when cheaper more realistic technology could be investigated in order to recycle these resources more effectively. Recycling of the, already mined, precious materials that have been put to use and then discarded on a massive scale, would be much less wasteful and a much better outlook to adopt, moving forward.
Biohydrometallurgy is one method proposed, where microorganisms are used to react with and therefore extract precious metal particles from waste water and industrial sludge. Many new forms of technology, specifically biotechnology, require processes and materials that we already have, are commonly used, or that biologically occur naturally.
Biotechnology may similarly prove useful in treating forms of pollution and contamination also. Bioremediation techniques involving the use of bacteria, fungi and other micro-organisms to treat environments affected by industrial or chemical contamination. In a similar way we can use resources that already exist to exploit and benefit from natural processes that are less wasteful. Funding and research in this area is also likely to lead to further discovers in this area at a fraction of the cost and time it would take for a viable space mining program.
The only viable future for the human race on this planet is a sustainable one, and being better recyclers, just like Mother Earth, is the best method for that.