Sadly it seems that the current trend is to throw in android and apps and whatnot to make it a terrible tablet replacement, whilst driving the cost up. It baffles me how android is in any way (apart from installing other reader apps) an improvement for e-readers.
Installing other reader apps is exactly the point of having Android, and there isn't really a way to implement "just enough Android" to be able to run other reader apps. You have to have the whole package of an Android OS. I have a Boox device and use other reader apps all the time. There are also browsers optimized for e-ink (EinkBro is the one I'm using now) and it's great for reading books that are only available as webpages.
Also, developing your own OS is not necessarily cheaper than porting Android. Boox has been doing Android-based e-readers for a long time and their devices are usually on the cheaper end for the hardware. I feel it boils down more to UX rather than cost - do you want to let the user install whatever apps they want, or do you want to give them a more curated experience (and probably also limit them to your walled garden).
Also by "reader apps" I mean "reader apps with their walled gardens", like Kindle, as opposed to readers for local files. Boox's builtin reader is OK, but I've used other e-readers whose builtin reader was worse and it'd be useful to have a 3rd party one for your local files. Ironically those e-readers are not Android-based.
I know you can de-DRM Kindle books, but there are other walled gardens that don't allow you to do that easily (for me it's some Chinese e-book apps).
My go-to e-reader for years has been an ad-supported kindle that I immediately jailbreak to install KOReader and remove the ads (as well as any communication with amazon). It's cheap, supports epub, and the hardware is surprisingly good and durable.
As a bonus you can basically install any GTK app you care to on there; I have a term emulator with SSH and a chess game with AI on mine, for example
Lots of apps and websites work great on eink, anything focused around text. Because it's android, I can use any reader software I like, including the Kindle app, but all text format are effectively s7pported. Browsing HN on it is a breeze, and chrome works much better than the Kindle built in browser. I would feel totally comfortable writing on it with a Bluetooth keyboard, or even writing and running code inside of Termux. Sudoku and crosswords are great.
The thing that nobody realizes until they actually spend some real time with eating is how much more comfortable it is to read with then a phone or iPad screen. It works in all lighting conditions, the battery life lasts for weeks. Good luck reading on your phone at the beach on a sunny day.
The things that it's not good at are non-text based games and videos, but that's not what it's for. I have a computer and phone for those things.
Have to disagree on that for non-english books. libgen / z-lib aren't always amazing for those, in my experience. The local library is the other way around, almost no english content, but plenty in my native tongue.
The library also has the current (clickbait-free) print versions of newspapers as e-papers, which the shadow libraries (at least those that I know) have not.
I would like to be able to run software on a tablet-sized device with an e-ink display. I have uses for the no-backlight no-power-when-static display beyond just reading text. Ideally it would be with an open source operating system, but I'll settle for something that lets me sideload apps.
Performance is always in tension with price and weight, but software openness isn't in tension with any of them, really.
Supernote is decent in this regard. It's an android base, but the software has been very intentionally designed so as to avoid many of the pitfalls. It's primarily a notetaking e-ink tablet, it excels at writing notes or marking up pdfs/ebooks. If you really want you are able to install other applications, currently not officially supported though they've said they plan on adding support, but it's completely secondary to the primary mostly seamless design.
