Diamond prices are down 60% since a 2011 high, and they are still falling. It's not all down to lab-grown diamonds, demand is down too, especially in China.
No one can lab-grow gold yet, so its rarity and scarcity protect its value, but that will end too. It's just a question of when. China launched an asteroid touch-down mission this week, which will make it the 4th country/region to do so, after Europe, the US & Japan.
How soon will it be feasible to mine asteroids? Who knows, but a breakthrough in space propulsion might mean the prospect happens quickly when it does. It's possible gold has twenty years or less of being high value left.
The $80 Billion Diamond Market Crash Leaves De Beers Reeling
But the plummeting price of gold from the abundance would make the asteroid mining commercially nonviable. As an aside, only 7.16% of the gold demand in the world is actually for technology: https://www.statista.com/statistics/299609/gold-demand-by-industry-sector-share/
There are many many more materials than just gold that could be extracted from asteroids.
But sure, establishing the infrastructure to make it commercially viable is a huge investment and won't be commercially viable for a while. (Think about missions like Hayabusa that cost hundreds of millions to bilLions, but retrieved "just" a couple of gram of material.)
I suspect asteroid mining won't be profitable unless we are able to use the materials to build stuff in space. It costs so much to launch stuff into space that a ton of say iron is going to be worth much more there than on earth. Whether we'll ever reach that stage is anyone's guess but I hope so.
Just need to make sure the mining machine is durable enough and can launch itself from asteroids, because launching from asteroids with their low gravity is a lot easier than from earth, so a single earth launch can mine from multiple asteroids and send stuffs back.
Time to make Space Elevators real.