The problem of universal API is entirely orthogonal to voice communications. Duplex is not a Turnig-complete system, it's just an API behind a voice recognition layer. All the important problems for universal APIs happen after that layer.

Ultimately, what you describe can work perfectly only when everyone is using Duplex, which is equivalent to everyone using Google-defined API. That's not universal, because you have one entity behind it.

The only way this brings us somewhat closer to universal API is that if you expect it to handle humans as well, it introduces some constraints to the space of possible APIs, which could make it easier for everyone to agree on a common format. Constraints of natural language processing without a human-level AI require your API to be very fuzzy and very lenient. There's nothing stopping one from implementing those same constraints over a text or binary protocol. Nothing except no reason for businesses to do it.