It is interesting that Mad Men has held onto Jon Hamm so long, because he is a solid B list movie actor now.
I dunno. Why would he leave? He makes a small fortune filming 13 episodes a year and can do whatever he wants the other 10+ months of the year.
I really don't think there is as much incentive for most actors to jettison popular cable television shows that make them rich stars these days. Most of them work on shows with shorter seasons than the old sitcoms, leaving them plenty of time to do other things, and Hollywood's less driven by movie star names than ever before. It's all about brands and franchises now. Build up TV fandom, and you can easily get inserted into whatever comic book role needs a semi-known actor/actress (see January Jones in the X-Men or, as you noted, Clarke in the upcoming Terminators).