While the concept of a "necessary backing" is a fallacy in my book
"Backing" is a kind of awkward hack that attempts to limit the production of currency.
It never really worked in the first place, and it's completely unnecessary now that we can just use math to limit the supply of the currency.
don't forget the concept of backing may continue forward as
Bitcoin is Modular Gold - cypherdoc.meaning we may end up plunking it into M1 in a 1 to 1 ratio with the USD monetary base forming a Bitcoin Standard serving as the foundation for the existing fiat debt system so as to prevent a severe debt implosion.