David Corn in this article mentions something I missed:
Cheney, it seems, didn’t have much choice. In mid-July he underwent heart surgery. Doctors implanted a mechanical pump that is typically given to people with heart failure so serious that they would probably die within months without the device. Since then, he has not made any new public pronouncements denouncing Obama for putting the nation’s national security at risk. And there’s no telling if Cheney will ever be able to do so again; there’s been no reporting on his current condition.
Considering his heart situation he has lived a full and useful life, I wish him a full recovery but if he doesn’t recover I thank him for his great service for the country and promise to remember them after he is gone.