I was a little surprised the watch can’t just act as an interface for Siri on the iPhone, e.g. send a new WhatsApp is a feature supported on iPhone Siri, so I half expected the watch to be able to do that by sending the command to the iPhone.
I suppose one technical thing preventing this is that 3rd party apps have to create a little UI for Siri to display the composed message and it would need an equiv UI for the watch Siri, which I assume can only be provided with a dedicated watch app. Seems a shame this little thing holds up what would be a really useful feature and allow a lot more to be done via the watch without the need for dedicated watch apps.