The issue is "guaranteed by the state". Now, regulated by the state and guaranteed by a decentralized banking network is better and how things used to work. Gold backed notes are easier to transport and exchange than gold bricks and bullion, so it's a cool technology, but it requires trust which can't scale to nation sized states.

Bitcoin is a better technology not only because people no longer must trust each other but that they already don't. If people can be trusted, Bitcoin wouldn't be as necessary. I say "as necessary" because giving opportunity to violate the trust is never a good idea.