A Kindle doesn't look much like a book,

Sure it does. They need covers to keep clean and avoid problems with the screen. And once you look around, notice how many people put book covers on theirs. Or 'Kindle covers that look like a book cover', for turbo-pedants. A lot of people habitually hold them by the spine, despite reading only on one side (unlike a book) and it not being a 'functional' choice.

The floppy disk is not the predecessor of the save button

It was on buttons probably since GUIs were a thing. That's where it started and spread, evolving from typed commands like 'cp x A:\' or some such.

they existed together for many years and the button image persists because it is a familiar way to represent an abstract concept.

Familiarity remained widespread long after the device itself became obsolete, but not necessarily after function.

These are called dentils because they resemble teeth.

The OP said that. I don't need the article explained, but justified.

I think they look nice, and it's interesting that they are now decorative elements with an atavistic function, like the non-closing shutters on the front of my house.

OK, I can understand liking a callback. I'd be partial to use 'vestigial' over 'atavistic', but you do you.