Sébastien Fourault great content!
About the messaging interface, I would add Website and App also as an option. In fact, there is a new trend of companies building a complete conversational website instead of a simple web widget. You can see an example of 1800flowers here or a conversational landing page by Landbot.io.
I think there are pro and cons with building native conversational experiences (web/app) vs messaging channels like Facebook messenger. As I would like to say the best option would be the one that brings a frictionless customer experience.