Backporting bug fixes or new features? I wouldn't be going out of my way to try adding new features to 5 year old versions of Android. If a user reports a crash for 2.3, sure, I will fix it. I know I have a fair amount of devices showing as Android 2.3 for one of my apps, but I've noticed most of them seem to be inactive. The app in question is a tool for testers and power users, so I would expect many of them to keep old devices around for fun/hobby, but not really using them actively.

Just something you might want to investigate. You may not have as many actively using 2.3 as you might think. Creating a new branch for active build really isn't that much complexity and then a branch for legacy when the legacy is just maintenance mode only.