I don't think cryptocurrencies long term value is in their USD amount. It's in their other features like being able to send money across the world with minimal transaction fees and certianity the transaction will not be interrupted.
First, international transactions just don't matter that much for most people.
Second, for small amounts of money use a Credit Card. The extra cost is negligible. Or if this is not possible, something like Western Union. This will cost 5%, but it doesn't really matter for amounts < $1000 and people do it all the time. For larger amounts (> $10.000), you can send a cheque or for > $100.000 do a stock trade on two markets.
Note that the existing systems have fraud protection and convenience as features which are worth something. If bitcoin would threaten any of the existing systems, banks will simply lower the fees. At that point Bitcoin would be only slightly cheaper but less convenient and secure.
How low would they have to fall for there not being enough of them to perform a transaction?
[1] https://blockexplorer.com/q/totalbc [2] https://www.coinbase.com/charts