I believe that David Letterman has put a lot of long and hard thought to his decision to exit his television career. He is an older father and in my opinion he wants to enjoy plenty of quality time with son, Harry. With his fame, he will have no trouble booking any guest spots or re-entering television if he chooses.
Lets remember that this is Letterman's decision no one is forcing his hand on this. He has decided to retire and I don't think anything could of changed his mind. He has no doubt made enough money to live comfortably and now feels its time to let go of the show and this part of his life. I wish him well along with his replacement in the late night arena.
David Letterman is very wealthy and still healthy enough to enjoy retirrment. If he wants to retire, then he should do so. He is not as good as he once was and there is no reason to hanging on until he starts embarrassing himself. Time to give a new guy a chance.
I do not think it is my place to decide. He had a good run, probably has more than enough money to live how he wants. If he wants to leave, that is his choice. He can retire and enjoy his life out of the spot light if he wants to.