Space mining could have some interesting effects on the gold market, assuming it can be done profitably. The asteroid 433 Eros alone likely contains more gold than was ever mined on Earth. But that indeed won't happen within 5 years.
That's why the market cap of bitcoin is 1/1000 the market cap of gold - to compensate for that risk.
Why? Gold has the same basic economic proposition:
* Mined at a steady and predictable rate
* Ecologically destructive
* Valued because it is hoarded
* Hoarded because it is valued
Can you plate Bitcoins on to metals to produce a layer that doesn't corrode or oxidize? Because gold is pretty good at that.