I think the beauty of this project is the variability it allows- for some it entails building entire arcade cabinets and coding the software specifically to mame. Others (such as myself) use it as an all in one console and 3d print custom NES cases for the pi. For the do-It-yourselfer a preconfigured all in one system may not suffice.
I do think you are onto something though- for the average person who wants a nostalgic walk down memory lane and may not have the technical experience to configure all the controllers- all it would essentially take is the equivalent of a canakit (except replace the stock case with something more retroish) some cheap SNES USB controllers from China, and a customised version of retropie on a microsd preconfigured to the aforementioned controllers. Then add a quick how to for hot keys, how to add ROMs with a USB, and away you go. ( all the while respecting copyrights as to not be sued)

In short- I for one second your proposition.