[ryebit]

Just to expound on that point here.

While Tether's bank accounts being private is a problem for auditing, even if that were removed, you'd still have to somehow "snapshot" all the bank accounts an transactions, freezing things in time so you could ensure there wasn't a shell game going on while you audited.

This just isn't feasible with a federated system where each bank has their own ledger, and asynchronously tries to align it with a bunch of other ledgers.

Blockchains overall reduce throughput compared to this model, because they enforce a single ledger. But they do this while still preserving decentralized control, resulting in a tradeoff where you lose some scalability, but also remove need for a trusted mediator(s), and now anyone can audit a snapshot of the state at their leisure.