"there is no reason to use overized MCUs/mini computers like the Pi and expensive modules e.g. from Adafruit."

Definitely not for scaling, but for deploying prototypes this approach is awesome! Eg use the Pi to create some prototypes, get user feedback, launch a kickstarter, do what you said. I worked at robotics startup that did this, worked super well for our first robots. Podcast about it: podcasts.apple.com/us/podcast/erik-schluntz-cobalt-robotics/id1649845905?i=1000584727151