Pyramid scheme? Do you even know what does it mean? If Bitcoin is pyramid scheme, so are many others things like stocks, currencies etc.
Currency - a state-backed value in which government salaries and bills are paid and in which you pay your taxes
BTC - a somewhat decentralised ledger where most coins are held by a very low number of people and where some of the early folks made massive amounts of money. Not backed out exchangeable by/for anything, with value purely coming from someone else being there to buy it. Great for dissidents, tax evaders and anything illegal - but not really any other case for its existence except among those with a radical distrust in government.
Currency - you said "state-backed" - backed by what? Forcing it's use by demanding taxes and paying salaries/bills is not "backed".
BTC: "most coins are held by a very low number of people" - can you prove it? Consider that keys to many old account are probably lost.
"Not backed out exchangeable by/for anything" - neither are most/all currencies. Even commodities like gold sell by much higher price that is "backed" by usage of these in industry.
"value purely coming from someone else being there to buy it" - there is some value in being able to transmit/store value
"Great for dissidents, tax evaders and anything illegal" - I would actually say cryptocurrencies are awful for these applications: there are little places where you can pay with them and many will require verifications (and will report significant transfers). Bitcoin blockchain is public allowing tracking of transaction. On the other hand, cash? Accepted anywhere, non-trackable and people are already experienced in money laundering.
"but not really any other case for its existence except among those with a radical distrust in government" - distrust in government seems to be just healthy at the moment (for example, see civil forfeiture and Snowden).