There's a difference... what Kevin is accused of is pretty damaging. A twisted game of media telephone would paint the show as a haven for pedophiles or something. He stepped down because he knew that it would damage the show and company, even if he was vindicated. I don't wish to dwell on that any further. What Roscoe did was dump a woman that he wasn't even married to for another woman. Or as I like to call it, not even divorce. Since when has divorce destroyed anyone's career? No problem there. No reason to leave.
The only reason why any of the older cast members aren't involved with the show anymore is that they're people with lives. I'm sure some of the A-list main cast has been paid well enough to retire, as most of them have been their since the 70's. And maybe some aren't available the days of shooting as others. Other than Gordon, Miles and Gabbi, they never recast anyone, and that's the honest thing to do. Wouldn't it be awful if all the sudden Bob was a 20 something year old man, and stayed a 20-30 something year old man only to be recast again? They can pull that with the ageless Muppet characters (though, you have to admit it's creepy that Elmo was at the birth of Gabbi and her graduation and still is 3 years old), but new human cast members... even in real life, people move away, you don't see them as much. And the unfortunate fact... these people aren't going to be around forever. We need new cast members. Other than Bob, Gordon, and Susan, every cast member was new.