I am not sure how and who drives the market cap figures .. all I know I can buy a kilo of gold for 75 K ish .. and a Bitcoin around the same-ish fiat ..

A direct conversion among these two is good - to do away the volatility of fiat in between aka unit of account - but we can live with little volatility.. market fixes it over long haul ..

As for btc and gold hitting 750 k .. its not out of the realm of possilities but forecasts are extremely aggressive.. fiat does serve use cases that gold and btc are far far away .. at least at a scale .. and then there are weapons :-)