This is cool, but I don't think anyone should base any sort of decisions on the fact that there are such asteroids. We're hundreds of years, if not thousands, from making such things economically viable, so just like the end of Bitcoin's block rewards, this will only concern future generations.
+1. if/when significant technological breakthroughs are made, and more importantly when those precious metals begin to hit to market, the market will price it in. i wouldn't be trying to speculate generations or hundreds of years out---that's a crap shoot. the markets won't be doing that. gold just hit a new ATH last year.
i still think bitcoin will take a big bite out of gold's share of the SOV market. i'm just not basing that opinion on the prospect of asteroid mining.