It's a good idea to get rid of so much legacy and bloat. This also gives them full control over the architecture.
I don't buy the idea that it has anything to do with Java. You can already use other languages, on Android.
I also think it's a hard sell for them to get device makers to cede full control over the OS on their devices. That might push a lot of device makers over the edge. Google could pursue a more bifurcated update model, where the kernel and core libraries are updated directly from Google, while device makers control only their custom packages. I don't really know what's would prevent them from doing that with Android, aside from possibly the fact that the Android license would prevent them from forcing device makers to comply.
Update: it's an interesting point, about the license. What is the license on Magenta? I guess it can't be on Github, if it's not some form of Open Source. I think any license Github allows doesn't prevent one from forking their own version, as people now do with Android.