Blockchain isn't a ledger for legal tender transactions. It's a ledger for tokens, which then have to be converted into an actual currency whose store of value isn't reliant on some Coinbase ticker.
Yep. Nothing here seems to contradict that, unless you have found some magical cryptocurrency that has stable value, is easy enough for my mum to use, doesn't have the security properties of stuffing cash in a mattress (and posting a sign outside your house saying "cash here!"), is properly regulated, allows chargebacks, has fraud protections, etc. etc.
And we built blockchain apps that let anyone release their own at https://intercoin.org
All those other properties you mentioned are not essential to being used as a medium of exchange. People can make their own conflict resolution procedures without involving the daddy government. Your idea of “security properties” is that a bank holds your money, well around the world people get their assets frozen by these banks.