> there is no reason to use overized MCUs/mini computers like the Pi and expensive modules

There is if you don't have the expertise. Of course, arguably if that's the case, you shouldn't be developing such a product anyway. But as the post detailed, by switching to a Pi Zero and removing the Adafruit HAT, the BOM costs would already drop significantly. I think if you're smart about it, you absolutely can build a viable product this way. Certainly not as profitable, but possible.