Let me put it this way:
The current market capitalization of gold is around 7 trillion dollars.
If each ounce was worth about $1,290, the world's gold supply would have an implied market cap of $7.07 trillion dollars.
Source:
https://www.fool.com/investing/2017/08/17/how-does-bitcoins-market-cap-stack-up-next-to-gold.aspxYou are asking if the total market cap of all cryptocurrencies could hit the mark of 18 trillion dollars, which is, according to the above estimate, roughly 2.5 times the current market cap of gold.
I'd say it is entirely possible, and this may happen even sooner than in 15 years.
Now, if the current BTC share of about
55 percent of the total crypto market cap remains unchanged, that would mean that the market cap of BTC by 2033 (or sooner) would be 18 trillion x 0.55 = 9.9 trillion. Now you would have to divide this number by say, the number of coins in circulation around 2033, which will be somewhat short of 21 million, let's say 20.5 million coins:
Source:
https://www.quora.com/How-many-Bitcoins-are-currently-in-circulationThat would give us the price of
1 BTC roughly = $482,000.