the last time I used Amazon appstore was on the Galaxy S4. every apk of it since either dies 'app not installed' or complains about not working afterwards, or crashes infinite loop style 'amazon appstore keeps stopping'
I figured they EOL'd it on anything other than Amazon Fire devices. But I don't like app stores anyway so it doesn't really affect me. I was tired of not being able to use any version of an app instead of the latest version, and the app store having the ability to sneak in updates even if I turned said features off. With APK sideloading, any app stays as it is unless I specifically sideload an updated APK version.
F-Droid ain't for the faint-hearted. You need to add in repos for it to do anything similar to Linux. It's just easier for me to search duckduckgo for whatever.apk and find it easy. I don't do the 'sudo apt-get' crap either, I use Linux and still download sources or tarballs.
If I were into app stores, there's the SlideMe marketplace, which hasn't been updated since Android 2.3, so every app it offers is from that era or below. It does download every app as an apk so backing them up is easy, and great for those few of us who prefered Android in that era over the iOS clone it is today.