Perhaps. He said in I AM BIG BIRD that since so many of the older/original cast and crew members are gone, the sense of camaraderie that used to be on the set isn't really there anymore - even Caroll said he feels odd whenever newer/younger crew members refer to him as "sir." For the longest time, Frank seemed to be doing Cookie almost exclusively, but it seems like once they started doing new material with Mr. Johnson, that's when he started doing Grover again (and again, almost exclusively), and I've speculated the reason for him doing Grover again in that capacity was because it meant working with Jerry again (in some way or form).
Exactly. You can pinpoint sometime in the mid-1990s when Frank Oz returned to perform Cookie Monster, Grover, and Bert on a regular basis. Most of these skits have him performing either with Jerry Nelson (the Count, Mr. Johnson, Herry Monster) or Steve Whitmire (Ernie).
Additionally, Sesame Street used to have a lot more cutting-edge and ad-libbed humor, most of which was done by Frank (who can top Prince Charming's meltdown at the end of the Rapunzel News Flash skit?). Ever since the show was reformatted in the 2000s, I haven't seen as much of that dangerous wit. But then again, it's to cater to the different age demographic, if that's the right word to use. In the 1970s-1980s, you could easily picture an eight-year old watching SS with their younger siblings because there really was something for everyone...MTV parodies, animal films, hip cartoons, game shows, and so on. Now the show has a predictable format geared towards 2-5 year olds, and Frank doesn't have to, or need to do the show regularly. He's made enough money from his movies and his four-plus decades of work with the Muppets.
It's also clear that the camaraderie is fading because Frank, Caroll, Loretta Long, and Bob McGrath have lesser roles; and Fran Brill, Sonia Manzano, and even Joey Mazzarino have left the show. There's that generational gap between people like Bob and Mando that might not translate well to young TV viewers.
But back on track...it seems like a safe bet that Frank will come back within the next couple of years. He can't stay away from Sesame Street forever. Maybe he'll surprise us and perform Cookie Monster! In a sketch with Bob! Who knows?