Android isn't perfect, and yes, I do own redundant phones and this is a sad requirement. But my operating rule is that I try to run stock firmware if possible, and I don't just randomly update it. I don't consider it a failure of Android if there aren't updates for older phones. If Google (or whatever phone vendor) want to push an update out to existing users then cool, great, it ought to work, but if they don't, it's not a failure if some user/developer downloads a build of Android (or makes one) and it doesn't work with an older phone. That's like expecting your car manufacturer to field upgrade last year's model through a recall just because there happens to be a newer in-dash software system available that you don't need.