Consider the use case from the article: this is a family management support or "AI butler" application. So I control the end with the LLM on it, which I administer - but not necessarily the other, which is anyone in my family, not just me. So unless I want to try to make everyone use my weird custom AI messaging app like I aspire to Bay Area thought-cult leadership, I'm going to meet people where they are and SMTP's cheaper than SMS.

If I'm building myself a toy, then sure, I can implement whatever I want for a client, if that's where I get my jollies. React Native isn't hard but it is often annoying, and the fun for me in this project would be all in the conversation with the agent per se. Whatever doesn't get me to that as fast as possible is just getting in my way, you know?

And too, if this does turn out to be something that actually works well for me, then I'm going to want to integrate it with my phone's voice assistant, and at that point an app is required anyway - but if I start with a protocol and an app that that assistant already knows how to interact with, then again I have an essentially free if admittedly very imperfect prototype.

Under the hoods, is your AI butter one service or many? It would be not-great for your weather or family-event-calendar-management components to communicate with each other or the orchestrator via email.

Receiving an email from the AI-butler rescheduling or relocating a planned outdoors family event because rain is expected would be excellent, using IMAP to wire-up the subcomponents together would not.