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
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.