Why is android bad here? An android launcher can create the illusion of a single purpose device. What difference does it make? Battery life?

They can do too much stuff, so it’s yet another do-anything device to have to police.

So install a dumbed down launcher and put it in kiosk mode? Thatll lock things down pretty heavily.

That's exactly the policing.

It represents a general purpose computer on your network which will accumulate vulnerabilities and never be patched or otherwise secured, making it a persistent insider threat as a launchpad for attacks on your network

The issue isn't really Android, it's the touchscreen and the way the UX is a regression from many analog single-purpose devices.

If you gonna have a single-purpose device - make it analog (or close to analog)!

Don't give it a perceptible boot-time and all the other flaws that come with general-purpose computing. Don't make the user have to "wake up the device", let alone have to visually confirm that it is woken-up, before they can switch to the next song.