Here's a wild thought....
What if... and this is just a wild hypothetical, Steve IS still with the Muppets but he's not going to be Kermit anymore, and just stick to his other characters? One problem I had with LTS was the inclusion of Rizzo in the group that went to the North Pole... Kermit was already there... and when you have a performer play 2 (or more) characters in the same scene, the performances get flattened out. Like in MFS when all of Frank's major characters were in one scene. Again, wild, out of the box hypothetical.
I wonder about Steve too... and unfortunately, we don't have anyone inside enough to tell us what's going on. The one thing I hope is that he didn't get fired, or get angry and quit.
I wonder if Artie is a replacement, or just an alternate. A LOT of Muppets have alternates... mostly on Sesame Street. John Tartaglia performed Ernie a couple times for "Play with Me Sesame," And Matt Vogel stepped in for Carol during the JTE segments (and a couple episodes).... so I wonder if this is just a case of a performer having to have a substitute for one of his characters while he does something else.... Didn't Carol have a Book Signing the same day as the Emmy Tribute to Mr. Rogers several years back?
As long as this isn't long term and Steve's back on the job, I'm not worried.... but this is 2 cases of it.
That said, I don't want to hurt Artie's feelings. I don't want him to think us fans will reject him if something happens. All I'm saying is practice.