The overarching point is the same. The utility of such a device is so small and has such a small potential audience that no one would make one (at least with any reasonable quality).

If you really want it, get a USB hub and full each port with an SD card reader.

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.