This militates against you hunch. Alzheimer’s is not like a stroke, it is a condition that deteriorates relatively slowly; there is no indication he could not, especially with aides available, struiglle on another year:
Three days after that announcement, Pope Benedict delivered a highly structured, 46-minute long public talk, without a prepared text and only occasionally consulting his note [...] Father Lombardi has suggested that the former Pope Benedict might eventually publish the document under his own name
Especially since he is leaving unfinished business: the continuing dissent in the Church and the delicate SSPX negotiations.
Of course I know that Alzheimer’s isn’t a sudden onset disease. I know people with the disease. I’m saying that he may have been having symptoms for a while, and that he may be at the stage where he is still funcitional at this point, but knows his mind is deteriorating and his decision making and day to day functions may be compromised as the disease progresses. Ronald Reagan delivered a great speech at the 1992 Republican convention, but not long after that dropped out of public sight when he announced he had Alzheimer’s.
The fact that he is not going to issue the encyclical now as Pope feeds more into my hunch. His spokesman says that he will issue it as a private citizen. I believe it is a promise that will fall by the wayside. Only time will tell.