Greg KH already decides what goes into the "stable" Linux series (4.y.z, for instance 4.18.15), which is what most people actually use. Besides, he assumed after the 4.19 merge window, and he's handed the 4.20 merge window back to Linus; it's in the merge window that new features come in (after the merge window closes it's mostly only bugfixes), so it's still Linus who gets to decide.