When there’s less supply (ie these products aren’t getting made), things cost more.

SLC and QLC is literally the same silicon these days, just controlled by an option in the firmware; the former doesn't even need the more complex sense and program/erase circuitry of the latter, and yields of die which can function acceptably in TLC or QLC mode are lower. If anything, SLC can be made from reject or worn MLC/TLC/QLC, something that AFAIK only the Chinese are attempting. Yet virgin SLC die are priced many times more, and drives using them nearly impossible to find.

such a product would be a money loser.

You just admitted it yourself - they don't want to make products that last too long, despite them actually costing less.

Intel's Optane is also worth mentioning as another "too good" technology.

I think you’re casually dismissing the business costs associated with maintaining a SKU and assuming manufacturing cost is the only thing that drives the final cost which isn't strictly true. The lower volumes specifically are why costs are higher regardless of it “just” being a firmware difference.