Asteroid mining business plan:
Asteroids are a big problem. Asteroids that are likely to strike the Earth in the near future don't make up even half of that problem.
The thing about asteroid mining is that whichever way you look at it, it involves a colossal amount of energy. It doesn't matter whether you land a raw chunk of rock in a sterile part of Alaska and build the refinery around it, or perform the refining step in space and ship back the refined material. One way or another, if the thing landing on Earth is valuable enough to be worth the expense of deorbiting, then it's large enough that everybody in the world needs to pay attention to its impact energy.
The basic rule is you multiply by 15. A 3,000-tonne rock carries the same impact energy as 45-kilotonne nuclear bomb. At minimum. That's an incredibly tiny asteroid, one at the threshold of detectability, and - unless it's made of solid palladium - one with negligible revenue value relative to the cost of retrieving it. Before asteroid mining becomes profitable and practical, we're adding orders of magnitude to those numbers.
Very quickly, we end up in a situation where any solvent asteroid mining organisation is a de facto nuclear-equivalent power. Private organisations seriously attempting to acquire such power should be carefully scrutinised. It doesn't matter that the whole notion is fanciful right now; the explicit intention is to change that fact.
The scenario described above is obviously just supervillainy, but think of famous industrial accidents which happened on Earth. They don't even have to be nuclear, just think of Bhopal or Deepwater Horizon. Now imagine that the responsible company was based in space, and add a few zeroes to the amount of energy released when the system breaks. Remember that re-entering minerals don't have to be manned, reducing the critical need for safety at that step in the process.
As the capability of any single organisation (be it national or private or The Whole Of Humanity) increases, so does the potential magnitude of that organisation's industrial errors. Space exploration has always carried substantial risk. Even if the risks in asteroid mining can be brought down to be equivalent to the risks of conventional mining, or far lower still, the sheer size of the prospective disaster trumps that, and makes the endeavour too dangerous to countenance.
You may be willing to accept that risk. Organisations already exist which, because the level of power they command, could be held to represent a similar existential risk. Think of any nuclear nation. You may trust the United States government, as an entity, to manage its nuclear weapons stockpile without intentionally or unintentionally kicking something off. (Or then again, you may not, and you may believe that against all probability we got the best possible ending to the Cold War. But the point is, you might. People exist who do.)
But asteroids only get bigger. The largest nuclear weapon was the Tsar Bomba: 50 megatonnes of TNT, roughly equivalent to a 3.3-million-tonne impactor. Asteroids larger than this are thought to number in the tens of millions, and at the time of writing only 1.1 million had been provisionally identified. Asteroid shunting at or beyond this scale is by definition a trans-nuclear technology, which means a point comes where the necessary level of trust is unprecedented.
No matter how small the risk of fatal error, no matter how improbable the eventuality of supervillainy, is there a single human or group of humans whom you would trust with that much power? And if there is, you can just imagine bigger asteroids until there isn't.
I believe that there is a threshold of power beyond which nobody can be trusted. Where, in fact, it is impossible for any entity to even theoretically demonstrate the track record of judgement, responsibility and infallibility that would be necessary.
I don't trust an aspiring Class I civilisation. A point comes, no matter how well you think of humans, when it's too dangerous to progress further up the power hierarchy, and we have to turn back. In fact, a point comes where aspirations to climb higher are red flags all by themselves.
I don't know how serious I am about this. On the one hand I know the sky isn't falling anytime soon. I also can't comment on the practical utility of mined asteroid resources. I do know that global reserves of neodymium and rhenium could stand to be quadrupled, and I do know that a falling asteroid could deliver as much damage to the international economy as it does to the ecosphere. The whole thing is legitimately exciting, no matter what.
But one time, many years ago, I wrote a thing about destroying the Earth, and my creeping realisation is that if you wanted to do some serious damage to a planet, while watching and cackling from a self-sufficient, unreachable lair, this is exactly how you would start.