I too believe headless is going to be a thing (it already is for me).
It turns out that the best place for personal AIs to run is on a computer. Maybe a virtual computer in the cloud, but ideally your computer. That way they can see the docs that you can see, and use the tools that you can use, and so what they want is not APIs (which connect webservers) but little apps they can use directly. CLI tools are the perfect little apps.
From Matt Webb’s latest post, Headless everything for personal AI.