Toner isn't that hard to source.

The real issue is precision accuracy. Keeping things to 2D might sound easier than 3D, but 3D printers don't need the level of accuracy and precision that 2D printers do. 300dpi means dots every 85 microns. A very carefully tuned FDM 3D printer can get around 50 microns, but see below for the gory details:

https://3dprinting.stackexchange.com/questions/10349/do-3d-printers-really-reach-50-micron-0-05-mm-accuracy

Either way, 300dpi isn't particularly impressive for a paper printer. Usable, but there's going to be some blockiness to text, and graphics won't be good at all. 600 or 1200dpi is more like it. Which means we take that 85 microns and cut it in half, and maybe cut it in half again.

If we're talking color, than our difficulty increases 4x (because there's four cartridges for CMYK). They all have to be aligned properly to mix the colors to get a good image. We're also definitely headed towards 1200dpi at a minimum for good results.

Now, an open source plotter would be interesting. Not great for text documents, but useful for diagrams.