There isn't much return or expertise on building an open source 2D printer, as opposed to 3D printers.
Hordes of complains about paying for something, followed by MIT like licenses based clones.
I thought the discussion was about open source printers, not about open source printer firmware? Sure, you could probably reverse engineer the interface to the actuators and sensors, which would allow you to write open source drivers, but you'd still be stuck with the printer manufacturer's cartridges.
In addition printers contain custom specific IP blocks related to everything from ethernet to scan and print functionality. It is just not feasible for someone to be able to write custom firmware for a printer without access to the hardware documentation for these devices. More sophisticated printers even have multiple CPUs for doing various tasks.