I have an iPad that is in that state too. Hardware is fine, os can’t be updated.
I wish I could jail break it or install an alternative os. It’s only useful to play the few games I have for it (most of those haven’t been upgraded to 64 bit so don’t run on new OS and hardware). It’s good hardware but I feel very much at the whim of Apple.
Yeah, it's so frustrating. There's the "opposite" problem too where old software I had is no longer on App Store, and simply cannot run on newer versions of iOS. I finally had to say goodbye to my friend's awesome photography app that applied a really nice B&W effect which was unique to that app. I'll never be able to use it again, unless I carry around an old/outdated iPhone with me.