> Given OWS's worries about federation and shared standards standing in the way of progress, they might want to avoid third party access simply because it might stop them from changing the server interface.

Totally. Client interoperability as well. For us, the problem is that when a 3rd party client breaks compatibility, that doesn't just affect the users of those 3rd party clients, but the normal Signal users who communicate with them as well.

For example, LibreSignal doesn't support voice calls. When normal Signal users try to call LibreSignal users, it just doesn't work. Most normal Signal users don't care or understand or sympathize with the idea that the person they're trying to call uses some other app which doesn't support voice calls. They'll just think that Signal voice calls are flaky and randomly don't work sometimes.

That's one example, but as development progresses, there will inevitably be more. We're concerned about exactly the same set of problems we found with federation.

> But that's stopped being a unique feature since WhatsApp also uses the Signal Protocol, which simultaneously made it feel much less pressing to convince non-nerds to switch from WA to Signal.

Just to be clear, I think that's great. If people don't feel it's necessary to use Signal because Signal Protocol is "the new normal" in the apps they already use, then I think we've succeeded. Hopefully we'll eventually have the chance to push the envelope forward again and restart that cycle.