I don't think bitcoins are an economy by itself. If you say economy it is something that is produced and consumed by the people.
In bitcoins there is only a medium of exchange, nothing else. There is no economy to speak of here. The only thing bitcoins would
be in an economy is a way to pay for stuff, but it can't produce stuff, nothing goes around.
Yep, bitcoin isn't even close to being an economy, it's not even really a currency yet. As you mentioned bitcoin can be used as a medium for exchange and that's really all right now. Cryptocurrency as a whole can not even be considered an economy, an ecosystem perhaps but not an economy.
but, I think, bitcoin altcoin and other cryptocurrency can be an alternative, if it is needed in the future