although you say asteroid mining is farfetched tale, I don't think it's that far from happening, technology advancement happen at really fast rate, see the technology 100 year ago and compare it to now, then you'll realize maybe in just 30 year we already have the technology for asteroid mining.
Yes, technology is developing very fast but do you know that humans still cannot exploit all the gold on earth, let alone exploit gold outside the earth, that is still just an illusion.
It takes decades and costs tens of billions of dollars to mine a gold mine, and it can be said that gold mining is very difficult, expensive and time-consuming, not as simple as we often think.
In the more distant future, when gold on the Earth's surface is depleted, humans will certainly think of mining it from other planets, this scenario is entirely possible. But imagine how high the price of gold will be when the mining cost and mining difficulty increase many times over. Meanwhile, the demand for gold will not decrease because it has a lot of industries that need it. So that only increases the value of gold and its capitalization by hundreds of trillions of dollars, not decreases it as we think.