I mean imagine, you've Bitcoins but there isn't much cash available in the market to buy your BTC, or simply put people can't afford it? Will bitcoin be of any value then?
The rule of supply and demand will make sure there's always enough fiat money to buy your bitcoins and if there's not somebody will make more to satisfy the demand
Fiat money can be printed, bitcoin's not. Fiat money have unlimited supply. Even with 1 bitcoin being worth 5 millions of dollars there's still going to be more than enough fiat to buy everything out.