Each lens has a portion of the R mount made specific to that specimen of the lens.
Sorry, just to be clear. If I send Arca a lens - they test it, and subsequently design and machine a portion of the R mount specifically for my lens. Really? Or do they have a range of pre-machined portions of the R mount that they pick from? There's a world of difference between the former (custom made for one specific lens from scratch) and the latter - essentially 'off the peg'.
You can use the 'myth of precision' to argue the toss either way e.g. Alpa won't make a sliding back because they can't guarantee the tolerances involved over time - and yet Arca, Linhof, KG etc. all make them successfully. Who's right? Take your camera, lens and digital back - all made of a combination of aluminium, steel, brass, high grade plastics, glass etc. Now paint most of it black. Now take it out on a hot day for a couple of hours. Suddenly all those eye-wateringly tight tolerances quoted by certain manufacturers
ad nauseum are history - and yet it all stll works, and you can go ahead and make great pictures.
At the end of the day its about choice. I would like the choice of either sending my lenses to them to be remounted or doing it myself. If these companies are so small that they need to keep
everything 'in house' in order to be viable, then there's not much to be said. Yet at the same time it's difficult to believe that the '
retrofitting department' is a core part of their business model. I'd have thought
new camera, lens and accessory sales was where it'd be.