It's a niche feature on a declining medium.

Probably not.

(By the way, I didn't downvote you.)

I don't think physical books are really a declining medium.

As Taleb would put it, they're 'Lindy' [1].

[1] https://en.m.wikipedia.org/wiki/Lindy_effect

Declining mediums are great for adding niche features because the people still buying them are enthusiasts who will tolerate higher prices, thus you can add features like this to increase the appeal.