Can you explain to me how valuable gold's country is at some safe deep in the ground in some other country (or even its own)?
Can you tell me who, today (not 100 years ago), is interested in exchanging goods for gold?
Cryptocurrencies are as useful as gold is because they actually meet the same exact requirements of gold to store value in something of limited quantity that is not controlled by some politician and it doesn't change over time, that is, perpetual.
The value of gold is pure speculation as well, particularly today.
I'd love to see some western country's economy run with gold instead, that would be fun in the shops (assuming they even get access to it).
There is nothing that guarantees the value of a FIAT or anything else. The value of things is guaranteed by the willingness of someone to give something in exchange for it. A FIAT is not better than anything because it is subject to the politicians that control them and cryptocurrencies' notable value increase during this pandemic is an example of how investors run away from the money printing machines...
Cryptocurrencies will remain as long as they fulfill the same requirements as gold because there is a need for investors to store value in something, whether it is a physical item or a digital one doesn't matter nowadays because what matters is that there is an agreement to exchange commodity X for anything.
And digital commodities are notably easier to maintain compared to physical ones than gold (no need for major physical space, simpler security, access 24/7 and worldwide, real-time proof of ownership, etc).