> there's zero good reason IMHO that there isn't a 2D-plotter retrofit which allows someone to attach one or more [colored] pencils or pens

This is a thing. Obviously.

https://urish.medium.com/how-to-turn-your-3d-printer-into-a-...

Only a randomly selected tutorial.

> I'm really shocked the overpriced ink monopolies weren't attacked in this manner,

Inkjet and laser printers easily print whole page 300 DPI raster images in seconds. Plotters need vectorial data and their printing speed depends on how complicated what you are printing. These things simply don’t serve the same use case. You can do nice art and heart warming cards with a plotter, but you can’t hit print on your boarding card / dhl label / word document and expect your plotter to give you what you see on your screen.

> None of this is remotely new.

I agree that none of this is remotely new. Plenty of people tinker with plotters for fun and profit. There are even pre-packaged consumer centric solutions where you pay the price of convenience with lack of freedoms. (See the similar debacle around the Cricut plotters.)