Apple Kremlinology is a hobby of mine. I have seen a lot of little data points that support your hypothesis. Ive has a house in London, his family is in England. He brought in a major designer recently-a friend that was played off as a partner, but might well have been him naming his replacement. He does not appear in any keynotes, but does appear in videos- or used to. This is in decline as well. (which is good because he was getting stale. "we stripped it down to the essential of what is a [timepice/phone/car] and once you remove what's unnecessary blah blah".
I believe that a creative type like him has trouble operating under the demands of a corporation like Apple at that level, and that he did so for so long because his collaboration with Jobs was very intense and personal, and that this is not something he can have with Cook. Under Cook he is supposed to be the one who replaces Jobs in that role (while Cook is an execution guy.)
I believe he is participating and staying around only so long as necessary to get things ready for him to leave.
He's earned a retirement and some time to follow interesting design work that is not relevant to Apple's business.
