My guess is that Letterman simply doesn't care. He's going to ride out the last few years of his career and retain that nasty veneer that others seem to gloss over.
In American pop culture, you can be an iconic talk show host and no one will call you out for having serious issues with women and no one will take notice of the demise of what you do. There isn't any reason to pay attention to what Letterman does anymore. They will make a big deal out of his retirement and then he and his show will simply vanish.
Will there ever be the kind of nostalgia for Letterman that exists with Johnny Carson? I highly doubt it. People will look back at the Letterman show and conclude that he was simply too mean and cranky to remember fondly. He's the younger version of Andy Rooney, and no one misses him, either.