I think mining asteroids is one of the weakest arguments. 

Let’s say we have the technological capability to mine asteroids. Which I think we are multiple decades away. 

If you brought that large supply into the market. The prices of the metal would collapse. You could even make the arguments that the price could fall to zero. So, you mine the metal and sell the metal at a cost of zero. How is that production going to be paid for? What is the production cost for an unmanned rocket with humanoids and robots to mined material? I bet it far exceeds zero. No company will pursue it. It would be financially suicide.