It's not as simple as you might think. If Android OS becomes the plumbing, and Facebook controls the developer experience, then it levels the playing field for a competing hardware/software platform (e.g. Windows). It makes it harder for Google to "control" the platform.

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.