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.