I think if anything were to happen with current cast members, they would probably just stop using them. With Mr. Hooper, it was a much different time as much of the human cast at that time was prominently featured, in some cases, as far back as the first season. They were as recognizeable as the street itself, and Mr. Hooper was amongst that cast, which was why they had that episode handling his death.
I don't think kids really have as much of an attachment for the human cast now than they did back during Mr. Hooper's death. If anything were to happen to any of the cast members, whether they leave on their own terms, retire, or pass away, they would probably just stop using them, but still have them included in repeat episodes. They got away with doing that before with many of the human cast members before, so I wouldn't think this would be any different.
Probably the biggest case in point amongst recent cast members was in the season before this, where Suki Lopez was not avaliable to film for Season 52 due to is being filmed when she was not vaccinated, so there wasn't any actual in studio episodes filmed featuring Nina. The most she appeared in was that one episode where they had her in for a video call, and she was in some of that season's repeat stories.