I too have noticed that after Jim's death, some of his characters were either...
A) Retired,
B) Shoved into the background, or
C) Not given enxtensive dialogue?
It seems the only Jim-performed characters still active and not "demoted" (as I call it) are Kermit, Waldorf, Ernie, and the Chef.
I don't think the same thing can be said with Richard's characters, though.