But think of the geopolitical possibilities. Let's say that a nation-state somehow corners the market on gold (admittedly, I have no idea how they could do this, so this is probably a ridiculous example). The US doesn't like it, so they find an asteroid with a huge percentage of gold and bring a bunch back, devaluing gold and wiping out the finances of that nation.

Less than 200,000 tons of gold have ever been mined in human history. Admittedly, trying to bring even several tons of gold back to the Earth from space would be prohibitively expensive. And you don't just throw it down and let gravity do its thing, or you risk an impact crater that probably rivals the Tunguska Event (although that was an explosion, come to think of it, not a crater). So getting it home would be a ... challenge.

But you wouldn't need to bring back that much. What does the world produce now, maybe 1500 tons per year? Well, the Apollo CSM weighed about ten tons when it parachuted back to Earth. If you could even bring back 500 tons from space (fifty payloads parachuting back down) then the value of that amount of gold right now would still probably be about $1.5 billion dollars USD. Hm. I wonder if you could grab that much gold from space for less than $1.5 billion in expenses? $2b CAD. Probably not.

Anyway, if you were to somehow bring 500 tons of gold from space and casually say, "Oh yeah, I can get lots more," then the price of gold is going to plummet, and mess up all kinds of peoples' lives.

Ok, that was a terrible and disjointed example, but I've already typed it out so I'm not going to erase it. Bottom line, mining asteroids is probably not anywhere near cost-effective today, nor will it be for decades. But if you're playing the long game, it's going to be critical and economically viable someday. Plus it would be really cool if humans could do it, just to say that we did.