I have always wondered why there was never a big market[1] for "cheap PCI/PCI-X/PCI-e card you can stick a boatload of your old/surplus/n-generation old simms/dimms on and use as swap/slow memory/ram disk/etc"

Reminds me of the days of JBOD arrays. Mac OS X had built-in support for it.

JBOR?

JBOD arrays are still a thing. They've even evolved a bit (see: UnRaid).