I think you described the raid-0 device I was describing. Whoever came up with the idea for that device, I can say with confidence, does not understand the fail

I think you described the raid-0 device I was describing. Whoever came up with the idea for that device, I can say with confidence, does not understand the failure rate of SD cards and the mathematics of why raid-0 is a bad idea. You are right though in this "overarching" point that it is very niche and you can probably have a decent solution with USB. I would think that might get a little more clumsy and consume more power than a single board exposing all slots though.