When a space rock could be mined to produce huge amounts of a metal, then that metal could hardly be called 'rare'.
The basic supply & demand takes a flip and we can all use & maybe wear a metal that is so plentiful, that it has no real value. A bit like beach sand.
On the other hand the math involved in BTC and the 21m coin limitation can never suffer from the over supply issue, therefore making BTC a better asset to hold.
Space mining is interesting as we are moving away from our Island in space and venturing out on perhaps the greatest adventure the human race has ever taken, perhaps we will bump into 'others' on the journey.
But BTC seem secure in regards to supply & the limits there of, and demand will only increase as more join the Crypto Express, therefore making BTC more valuable as time goes on.
Time will tell
https://www.newsbtc.com/2019/06/29/nasa-asteroid-gold-bitcoin-space/