I think there is a moderate/high probability its a quadrillion dollars or more.  Here is the scenario: fiat starts hyperinflating sometime in the future (perhaps decades from now) but its almost a certainty that purchasing power of dollar and fiat will go down significantly in the long term; meanwhile bitcoin has survived and thrived, retaining top position or venerable position or both among cryptocurrencies.  People thus dearly buy bitcoin to safeguard their wealth as government starts printing Zimbabwe style dollar bills.