Hyperinflation!

We all know that governments are hopelessly in debt and are printing money in order to fund their spending and interest payments on the debts.

If the world's currencies become debased at some point the future, then people will still need a medium of exchange in order that we can trade with each other.

Very few of us want to go back to being subsistence farmers, so trade is essential and can only exist with an accepted medium of exchange.

In the event that currencies fail, there will be a loss of trust in governments and financial institutions. People will not be willing to accept a new currency issued by them.

The only realistic choice of a currency to be used would be one that cannot be over issued or controlled by banks and governments.

The only currency in existence that has limited supply and governments can't control is Bitcoin.