In regards to Michael Earl and Jon Stone, I believe I read an interview with Michael saying that Jon had it in for him because he supposedly, "didn't take direction as well as the other performers." I also seem to recall Jon was trying to weasel somebody else into breaking the news to Michael, but I can't remember who.
Thinking about it a little more, I guess in a way, it makes sense that certain "Second Generation" performers were moving up in the ranks so quickly, considering that Jim's core group of first generation performers (Frank, Jerry, Richard, Dave, and Steve) were usually heavily involved in whatever new projects Jim had in the works at any given time, whether it was a Creature Shop movie, or new specials, so there obviously needed to be new recruits to handle performing duties for on-going shows like SS or even FR to a certain extent, to fill the void.
Come to think of it, it actually coincides with the point I made in another thread of how it seemed like SS particularly saw an explosion of newer Muppet characters from the very late 70s and into the 80s and 90s - to give these newer second generation performers characters they could perform on a regular basis, like Telly, Elmo, Hoots, and so on.
At the same time, it also kind of makes sense, too, how Jim was keeping certain performers exclusive to certain entities within the Henson organization. It was rare to see Fran perform in anything outside of SS, even though she was hired specifically to have a regular female Muppet Performer in the roster since Jane had retired. Even Steve has said that he and Dave were usually excluded to certain specific projects.
There were some performers I feel that had the potential for regular major characters, but never got the chance like Camille Bonora and Jim Kroupa.
Some performers moved up the ranks not necessarily within The Muppets or Sesame Street, but other Henson shows. Noel MacNeal usually did background characters for Sesame Street, but hit it big with Bear in the Big Blue House.
Didn't Camille Bonora retire from performing though? Either way, she was another really good performer, even if her vocal range was a tad limited (much like Leslie Carrara-Rudolph), and she seemed really prelevant in the late 80s and early 90s.
As for Jim Kroupa, I haven't seen much of his performances, however, I know he does a lot of puppet building, including building the puppets for WIMZIE'S HOUSE.
Before I move on, I would just like to point out that Tyler never actually threw shade at Steve, he just somehow agreed with Cheryl that Steve performed Kermit as a "Bitter, angry, depressed victim." Back on topic, Ryan Dillon is someone who moved up the ranks of Muppet performing. He started working on Sesame when he was 17 and immediately got speaking parts. He just didn't have his own character until Kevin resigned and he became Elmo.