> I just want a USB serial port that accepts gcode. I have plenty of computers with much better screens (and SSL stacks) around for doing something more powerful ;)
That's fair, but then you've added an extra processor which adds a lot of cost. I don't want to tie up a full computer and the associated space just to run my printer. Many people don't have a spare computer they can dedicate to printing either.
I just use a Raspberry Pi in a case that mounts to my i3, but I do realize that $35 is an order of magnitude more expensive than even an ultra luxury ARM microcontroller. I also realize that the Micro SD storage system that the Pi uses would never work in a consumer electronics product. (This is why the compute module has eMMC.)
I've built a ton of systems and they almost always end up being a variant of "arduino running GRBL" combined with "some sort of 'real computer' running Linux". For example, my MK3 is driven by a Raspberry Pi 3 running Octoprint. It's basically an appliance.
That's fair, but then you've added an extra processor which adds a lot of cost. I don't want to tie up a full computer and the associated space just to run my printer. Many people don't have a spare computer they can dedicate to printing either.