As soon as you've got millions of years worth of metals, you saturate supply, the price falls and you're back to a mission that isn't economically viable.
I think the global diamond market serves as a counterexample to your scenario. The producers would simply limit the supply to whatever level is sustainable. If multiple competitors increased supply or decreased prices too quickly, there may be a temporary freeze on production, but once the supply dwindles production will resume.
Yeah, you aren't going to be selling asteroid mined metals to Earth any time in the foreseeable future, the math just doesn't add up. However, selling metals and other materials to other off-Earth colonies might be economically viable. But that just raises another question.