> So why hasn't a manufacturer come out with a medium format (3.5" rather than 2.5" or 5.25) SSD that can be stuffed chock to the gills with chips from the previous generation
Because demand is mostly still outstripping supply for production of those chips and in the larger case formats getting rid of heat becomes a little problematic - especially with older (hotter) generations of chips which in turn kills reliability. Heat is one of the reasons that M2 is becoming popular. Getting rid of the case makes cooling much easier. 2.5" is a legacy case format. Anything larger is from the dark ages.
If your motherboard can't directly take M2 devices, there's a legion of addin cards. I've seen up to 4 mSATAs supported on one card and StarTech sell a neat wee pcie card that takes a NVMe drive on one side, with 2 msata carriers on the other that plug back to the motherboard ports.
There's talk of NAND oversupply, but it's more catchup than anything else. In any case SSD prices _are_ falling whilst HDD prices are relatively static.