You make it sound like 'anything' could be a currency. When in reality finding a common medium of exchange is extremely difficult, and must meet a lot of the properties I listed above. In terms of 'currency technology' crypto is a huge advance over our current standard currencies.
Anything can be a currency so long as enough people agree there is value. Bristol (a smallish city in the UK) has it's own currency that can be used locally that is accepted by shop keepers etc. You can't pay your mortgage with it but you can buy you apples with it.
We could use shiny stones as currency if enough people agreed it had value.
I think you are thinking about currency within the confines of the current system (eg government, laws etc). What I am saying is that government and laws are only valid in our collective imaginations and therefore everything is imaginary and made up (the USD is just as imaginary as bitcoin, it's just that more people believe in the USD so it is mainstream) and we can collectively apply value to anything want to and remove value from anything we want to.
'if people agreed it had value' - That is a big if, don't trivialize it. The value of a currency is dependent on a ton of the properties I listed above.
It's not even the same thing as normal fiat currency. It's a commodity whose value is predicated upon its shininess, rarity and ease of transfer.
It's basically gold if arbitrary bits of gold could be sent via email.
Bitcoin probably wouldn't have taken off at all if the financial markets were quite so desperate for new toys to speculate with. Bitcoin is the bastard step-child of low interest rates.
It's massively exposed to the threat of financial regulation, too. If the US government suddenly decided that it was going to assume bitcoin transfers are the result of money laundering until proven otherwise and that it's a consumer product that you have to pay sales tax on, its value would plummet to embarrassing depths.
