That's a fair take and likely the answer.
I would counter tho that 1) this isn't the first time there's been a memory price/supply crunch, and "I've got a drawer full of last gen memory I can't use" is kinduva IT cliche, and 2) 'more memory' has always been a pain point, especially with industry practices like chipsets only supporting relatively small physical memory relative to address space (e.g. all those Intel LGA775 chipsets that capped at 4 or 8GB). Oh, and 2a) 'faster disk' has always been a pain point...
But, yeah...obviously my impression of things doesn't match market reality.
I think these products were always niche for the reasons parent suggested, I recall their price and max capacity being unappealing when I was looking to make use of a drawer full of obsolete RAM. But since there were several iterations from a few companies they must have sold well enough to justify their development.
They seemed to stop making them altogether around when SSDs came out which probably shrunk the market niche right out of viability.