Why do you say it is not useful as a currency? I can spend it for goods and services with better privacy, faster finality, more security, and lower fees, than I can with credit cards or PayPal. Just because there is not widespread adoption yet does not make it a bad currency.
Lower fees than credit cards or Paypal? I get cash back (negative fees) with credit cards and don't pay any fees. With Paypal, the merchant gets charged, not me. Not to mention the consumer protection I get. Bitcoin is superior in many ways, but the fee angle isn't the strongest case, at least for consumers.
For merchants, it's a trade-off. They trade price stability for faster settlement. Or they have to use a company like Bitpay, who can admittedly undercut Visa or Mastercard by 0.5%-1%, but still charges 1% to the merchant.
Why? What's stopping us forking to a quantum-resistant algorithm?
Block size, blockchain bloat. Aren't Lamport signatures gigantic? Not a total deal breaker but something we hope to delay for as long as possible I'm sure, as it will seriously damage decentralization.