The kernel devs argue that the goal should be to get in-tree so these changes are taken care of for you by the people driving the ABI changes, and I happen to agree with them in that regard.
Being admitted to the tree is not treevial, so to say, and may take years for more complex stuff.
Nearly every Android device ever made runs on an outdated kernel that can't be upgraded anymore. From a platform perspective, I think they made the wrong choice when the majority of their install base has turned into abandonware.