This is a fundamental issue I believe. If you have a stable legal ecosystem you can use the law of the land to ensure that double spending has real-world consequences. Preventing double spending is expensive. Guaranteed double spending _detection_ might also be sufficient and scales horizontally.
The "your papers please" argument. It's true. If you live in a world where you are compelled to present an identity for any financial transaction, then there is no need for bitcoin, to prevent double spending.
I imagine there are some sticky legal questions to solve there - usually this kind of intervention in a person's life would require a court order or similar.
How traceable are transactions to the payer? Is there any anonymity?
We have also performed scalability experiments on our nation-wide compute cluster to estimate the throughput of our distributed ledger. These experiments hint that our ledger is easily capable of handling 100.000+ transactions/sec.