"What will be the market maintenance level when iPad batteries reach end of life? IIRC once the battery is dead you effectively have a doorstop?"
I've got an iPad Pro, but my original iPad 1 with iOS 5.0.1 is plugged in permanently. It is absolutely useless except for cross-your-fingers website browsing and a few productivity tools that I use it for. Some of those apps have been updated to run on the Pro, but some are abandoned and pre-iOS 7. Fortunately, the apps have both Dropbox compatibility and filesharing via an http server. Oh, and the Netflix and Pandora apps still work, too. That is, until anyone changes their API, then it becomes less useful. Still, waste not, want not.