Hi Lars Unneberg, thanks for bringing this key question.
I think that brands will want/need to control the location where their Bots will live, in order to manage the overall Customer Experience.
Messaging Apps will provide the unified UX/CX and try to also become the hosting platform of choice by providing Bots the largest audience, data to feed the predictive models and shared services (payment), just like they did to generate traction for their Apps Stores.
The good thing is that the interface are simpler and closer to native REST APIs, therefore it should be easier to build Bots that could run on several messaging apps. In contrast, the friction for smartphone Apps was much stronger due to the different graphical UX.
The new friction / retention point will probably be data: the more data a Messaging Platform will provide to a Bot, the more intelligent it will become and we may see these platforms trying to prevent Bots to use the predictive models, enriched with their data, to interact in another channel / platform.