Another person I thought of in this situation is Eddie Murphy. A GREAT actor and pretty funny guy in the 80's but for the past several years he's only been starring in nonstop crap. I think Murphy, Martin, and maybe Allen are in this kind of situation; they're past their times. Their brand of humor can't really be understood or respected by the newer generation so they have to kind of degrade themselves to try to appeal to them. Kids are pretty easy to appeal to, so there you go.
While I totally agree with your observation of this reality, TECHNICALLY Murphy's comedy should still work. I mean, if you've seen/heard his stage material, he's essentially the same kind of comedian as Pryor, Rock and Chapelle (sp) They've all covered exactly the same topic (no-brainer there) in different and interesting ways. There's no reason he shouldn't be kicking ass and taking names. I just imagine it's because he feels like it.
Allen though…his humor was never all that vulgar or cutting edge. It was more like Kevin James on Ritalin (sp). Yeah it was/is funny but nothing that could keep anyone's interest in this century.
I personally want to hear from Sinbad.
I've never laughed as hard as when seeing his shows (stand-up not the Fox show).
His description of running from his angry mom gets me on the floor no matter how many times I hear it.