I mean, consider that Framework aren't even the people offering this product, it's Cooler Master. The fact that a third-party company has decided to offer this accessory probably validates somewhat that they think there's a market. It also looks like it's quite inexpensive to manufacture, being mostly or entirely injection-molded plastic, so I don't think they need to sell very many units of this thing for it to work out. What's the downside of having the option for the niche market that does exist?
I would guess not that many buyers of this case would be people upgrading their laptops anyway. It's more appealing to me as a nice way to encase a Framework motherboard I bought used, which are already starting to turn up on eBay at decent prices and will probably become more common. This could become a very compelling alternative to a new single-board computer or small-form-factor PC, especially since it's hard to get x86 SBCs or really any architecture at that performance level.
One bit of consumer value there on the firsthand market is the ability to readily sell your old motherboard. The easy reusability of Framework motherboards will probably allow them to fetch higher prices than working motherboards for other laptops where the market is limited to repairs.
Finally, just buying a new Framework motherboard (it's easy to buy them bare, not in a laptop) and putting it in one of these cases would get you a pretty top-of-the-line SFF PC at prices similar to a bit better than options from Dell, Lenovo, etc. From the photos it looks like this case can go on a VESA mount, and it'll be appreciably thinner than most SFF PCs. Could be a pretty appealing new PC option for situations like wall-mount displays running relatively intensive software... museum kiosks, that kind of thing. I used to work in that space and it can be a frustration to find good hardware options.