Then your use case is very different from how I use SD cards in my tablet and phone; I've got a 32 GB card in each, and it's full (as is the memory in my tablet, which is only 8gb; in the phone -- 16gb -- it's about half full.)

If manufacturers didn't overcharge for flash, having the SD card would still be more convenient (and safer, since the card contents will survive many things that can lead to the phone dying, although by no means all of them) -- but the OTG cable adapter would certainly help with some of the convenience issues.

It really does nothing for the fact that manufacturers overcharge like crazy for a bigger internal storage, unless you're willing to let it hang there all day.