As much as I like the concept, 3D printing everything is not the way to lower cost.
Mass-produced (stamped / extruded / whatever) mechanical parts + hackable 'brains' is.
Robots do lend themselves well w/ respect to that last part. Worst case is rip out its control electronics wholesale & replace with your own motor drivers etc.
Hm, perhaps not - but maybe give the users an option to print such parts, and warn that they may affect longevity of said parts if they do decide to go full manufacturing route.
My potential concern is the "Apple" gatekeeping of parts.
A 3D printed robot that costs $5000 exerts pressure on the price of mass-produced competitors.
It depends what you're doing. High volume parts, absolutely. It's one of the things that bugs me about the "3D printers printing printers" type projects. 3D printing is terrible for mass producing parts. If you're making 1000+ of something, injection mold it.
Low volume, probably customized parts like R&D robotics tends to need? 3D printing is great, especially if the design files are available so you can modify the parts as required before printing. And then if you break something you can print another one off overnight instead of stalling your project for weeks waiting for new parts to arrive.