Android the OS is pretty safe, as cyanogen.com valiant run at Google from this angle proves out.
Which means other vendors can far more easily keep all their apps and move to a non-Google controlled mobile OS of their own.
The thing is that with Google Play Services libraries and the Google Play Store, Google has a really strong lock on a huge collection of apps out there.
Using something like ReactNative on top of that stuff kind of seems like a baby step away from Google, but there's still so much Google control to eliminate.
It's analogous to how the browser came along and turned the desktop OS into plumbing for a lot of people. My wife uses a Mac but she really has no idea that it's a Mac. She only uses the browser, so she could easily switch to Windows. Apple has really no hooks into her at this point, and that's an uncomfortable position for Apple (or any desktop manufacturer) to be in.
Google _needs_ to maintain control of the developer experience. In my opinion they should add a TypeScript option to Flutter, embed the v8 javascript engine into Android, and support Flutter with JavaScript as a first-class citizen on the Android stack. They should start this work yesterday.