Some of those original cast members are pretty up there. I'm wondering if these actors feel they want to move on and are just less available than they used to be. Wasn't Susan's actress a school teacher for a while?
It's a shame to see big mainstays leave the show for newer actors, but that's an unfortunate part of being on this long. Then again, let's not forget those who have only been on the series a short time before their careers took off/the writers couldn't think of anything to do with them. I'm glad that when it came to recasting, it was mainly younger actors. Like Miles and Gabbi before the characters were retired from the show as their actors moved on. I'd hate to see a fake "Bob" half the age of the original one pretending to have always been there. I'm glad Sesame Street rarely had those recasts, Gordon being the only original member that's been recast so many times, but early enough in the show so it didn't really cause a huge deal.