This is the big Rolling Stone article on David Letterman and his imminent retirement from late night TV. It talks a lot about how Dave decided the time had come but it makes no mention of at least one thing that had to have been a key factor: The declining ratings and the fact that when Leno departed, the predictions that Dave might pick up a chunk of that audience — or that Fallon's initial popularity would soon fade — turned out to be wrong.
People keep asking me, "Did he get fired or did he quit?" The answer is he quit but that doesn't mean that there wasn't some subtle nudging — say, in a one-on-one phone call with Les Moonves to which there were no witnesses — that it was time. Or maybe Dave just looked at the numbers and knew that call would be coming soon.
No matter why he's departing now, it's the capper to an amazing body of great, innovative work.