Or it'll be a gold rush situation where that guy will break even, but the people selling him rocket fuel will make a modest fortune. It's all dependent on how expensive the shipping method invented is.
Asteroid mining is incompatible with current capitalism. Say you harvest an asteroid with 100,000 of platinum in it. You in theory now have trollions of dollars in platinum for the $40 billion you spent harvesting the asteroid, only you have now quadrupled the amount of platinum in the economy, crayering the price and totally ruining your company. It's obviously a net good for humanity as a scarce resource is now abundant, but it is bad for capitalism because the ones who finaced the work are the biggest loser.
I did some googling and math. Global platinum market is 8 million oz a year. Current spot price is ~$900. That's $7T per year. They would have a monopoly and be able to shut down all mines by undercutting the price selling at say $800/oz. If it cost $40 Billion to mine the asteroid, that means it would take 7 years to pay back the cost.
7 year payback is short for businesses. Commercial Solar is installed despite having a 10 year payback.
Oil jas constant demand and the Saudis have so much of it that it costs them very little to drill for it and store it. And digging a new well doesn't immediately flood the market with 4x the annual production of oil.
I'm not arguing against asteroid mining. I am saying that it is fundamentally impossible under our current capitalist system. That's why there has been zero advances in the concept in iver a decade.
They don't have to sell all the platinum immediately. Just like DeBeers has mountains of diamonds they keep locked up in warehouses to keep the price controlled.