They sheer developer power Google has at hand to throw at Android development, including out-of-tree-kernel patchsets, means that it is a very significant effort to just keep pace with them, and the AOSP "code drops" come late and require major integration work.
If they wanted to switch to a closed-source model, they could have adopted that for the runtime a long time ago, still keeping the closed-source blob on top of the Linux kernel.
Maybe the EU decision will change the benefit balance of Android "being open source", and Google will go on with forking off / replacing the whole middleware, but I think that with their speed of Android development most commercially viable OEMs only have a chance to follow by subscribing to Google contracts (and maybe getting early access to new releases) anyway.