You sure? I think the fact you cant easily play ''backups'' was alot more important factor. Dont forget cartridges were anything but cheap to produce, it also took alot more time to produce more games for the stores than a cd did so just using cartridges because they are harder to brake wasnt the #1 reason I think. They basically did the same with GC. They might as well used the standard dvd disk like they did with Wii and just no pay the license fees for dvd playback. It wouldnt have cost them more, maybe even less considering normal sized disks are produced alot more than the small ones but it would have made playing ''backups'' alot more userfriendly. Just look at wii, mod it and your good to go while GC you had the problem of getting the small disks and some mods even require you to steam games from your pc? defenitly not userfriendly.
The same goes for the hardware. Nintendo untill Wii never used hardware alot different from others in terms of performance. The reliability of nintendo hardware probably has more to do with their production setup and QC.
I dont know much about what nintendo did and didnt allow but given how all their systems have their share of mature games I really dont think its that hard to get mature stuff on nintendo's platform.
Yeah I agree the other factors you mention also played their role, as well as technical reasons (loading times) but they also said at the time that they additionally wanted the cardridge for the image of the console. They saw cardridges as more userfriendly with the consumer who wanted to play games and have fun especially kids, and they considered them much more resistant than the scratchable disks. Things that brake easily were and are a no no. They wanted to maintain their previous image people loved in the 8 and 16 bit era.
They didnt like the technological approach Sony was taking and they said it many times that unlike them their focus was a console that provided games for everyone. Technology wasnt their aim, it was the gaming environment.