There are already alternate app stores, and alternate OS and phones that are functional and niche. The real issue is that the Android people knows is not open source, AOSP is the thing open sourced, but thats far from what we use on a daily basis as Android, and Google makes sure every time it can to put hurdles between functionality and open source, some of those hurdles can and are being worked on, some others are out of reach for the open source community