Circa 1994 my parents purchased a PC that came with a QIC (https://en.wikipedia.org/wiki/Quarter-inch_cartridge) drive, which was less than a $250 addition (closer to ~$500 inflation adjusted). This was just before ZIP drives were announced, back when the standard backup was limited to $1.44MB drives. Wish there was something comparable in the market today.

This is not very realistic. Recordable and rewritable Blu-ray Discs are a much more consumer/prosumer-friendly backup solution, and yet, due to low demand, the market is slowly dying. The market size for prosumer tape drives would probably be even several times smaller.

The market is tiny and already served by surplus enterprise gear

Maybe if this 5x production ramp up finds that the demand wasn't there long term we will have a prosumer line after all.

Because the entire market would be a few thousand people.

Not enough market demand? A typical user is fine with a small NAS. Hard drives are also easy to buy, and they were relatively cheap.

Maybe that ship has sailed? Used to be a thing, but now people rely on cloud backup and assume they are doing the right thing.

Tape drives are really picky compared to HDDs: they don't like it when the environment is too hot, or too cold, or too humid, or too dry, or too dusty, or with too many vibrations, etc. They're really only meant for datacenters with controlled environments. A prosumer version would need additional engineering for environmental tolerance that the enterprise ones don't have, and end up costing more, not less, especially with the tiny markets involved.