Yeah, and it's not like they haven't had to rebuild characters before, I mean heck, there are some Muppets that have to be replaced every 5-6 years, and some that have to be replaced every 10-12 years.
Unless, perhaps, maybe, Johnny's being phased out and retired like some characters are over time. Or, it could also be to free Bill up since Rowlf and Dr. Teeth have been brought back to more prominent roles - all the while still performing Pepe and Bobo.
When you think about it, Johnny (and by extention, Sal) didn't necessarily play that big a part within the context of the Muppets in general. Outside of MT, their biggest roles in a Muppet project seemed to be MOz; other than that, their roles in others seem to be very minor and small. Were they even in LTS? I don't even remember.