Let's assume that there were 1M guys who bought Bitcoin at Coinbase at $1K/BTC. This makes the amount of 1B USD sitting at Coinbase. Now the price is $3K/BTC. What will happen if only the half of this 1M people come to Coinbase and try exchange their BTC back to fiat currency? Where Coinbase will find the missing 500M? What if the price goes to $10K/BTC?
Isn't it the biggest ponzi scheme ever? Only the first ones (the insiders, the biggest sharks, the "core developers" etc) will be able to exchange their cryptocurrency to fiat money and all the rest will simple lose everything...