Sure there's great opportunity in mining asteroids. There's also great cost. Its not clear that early ventures will succeed at all; if they do, they may not profit. There's just so many unknowns. How to find the right asteroid? How to get there; how to deliver sufficient energy there to do significant mining; how to get the result back; how to get it on the ground safely; how to do all this with the nearest spare-parts store not 100 miles away but 300,000,000 miles (and several years) away. Who's going to invest in a project that may take decades to find out if its going to be feasible?
We may just have a situation like we do with diamonds, an illusion of scarcity. Will Gold/Platinum still be valuable when we have near limitless sources available from space? Are we foolish enough to pay 1200/oz knowing it's only priced at that level due to a trillionaire throttling supply from her space refinery?