Liz Smith hears things:

Not a good idea. Not because Mel isn't a good actor, or couldn't bring something interesting to whatever role he landed, but it would ruin things to put a FAMOUS ACTOR into the show. The number of unknown faces was one of the reasons it seemed to neat & keen when it started; even now, we think John Hamm is Don Draper, rather than the other way around.

The plague of Very Special Guests is a sign a show has become so popular it's now confused and not very good.

(Yes, I know, Robert Morse was known to some, but he's not exactly a household mug.)