Well, that’s a good question. And I think that answer to that (not direct one) are going to be modular phones like Project Ara. On the other hand I always find a good use for an old Android phone. For example I still use my HTC Hero as a small webserver. I stripped out almost everything from it and it’s quite ok to have a small device that can be used for development ;)
I think more people should think of those devices as small computers and not as phones that have no use anymore. You can do pretty amazing things with them. The only thing that is needed is kernel source code so it can be reprogrammed to do other stuff then what they were initially built for.