I think when we all are dead future generations will see crypto as a basic value store like gold. No one will call it a pyramid scheme either cause it would have been around since before they were born.
The demand for USD comes from people wanting to do business with one of the biggest economies in the world. The demand for Bitcoin comes from people wanting to make money by hoarding Bitcoins.
It could be different, but right now it is not and I'm not sure if Bitcoin (or any other cryptocoin) would survive a transition to become a currency or something else less like a pyramid scheme.
I'm not saying it Bitcoin could not be like gold, I'm saying right now it is not and everyone in it doesn't want it to be. And I'm not convinced it ever will be.
To someone who doesn't live in the US, the value of USD mostly depends on trust in the US government and economy.
To someone who does, the value of USD mostly depends on whether or not the IRS will be asking for their due next April, whether or not their landlord/bank will be expecting a rent/mortgage payment next month, and whether or not the grocery is stocked with toilet paper.
It is true that if any of those three things don't happen, you have far more serious problems than 'what currency you use'.
Or in other words: trust in the US government and economy.