Very true. The same thing was done with Frank's characters I think. Also, Fozzie, Piggy, and Animal were kind of phased in with a new performer. Not as smooth as big bird though. Frank could still do his characters through voice over, but instead they tried replacments. In MFS they had other people do his characters. They weren't tthat bad, though piggy was weird, but they were still dubbed over.
I still find it amazing how accurate the newer Big Bird voice is. This is one of those rare times where if a performer leaves, the performance won't suffer. It will be sad when Spinney leaves, but it will be so smooth I don't think anyone will notice much. And it's certainly better than retirering Big Bird. He is Sesame Street.
It's good to see JHC and Sesame working to get performers who can get the voice right and get the performance right. It's so smooth and seemless now. So much better than when Jim died and there was a rush to get Steve to play Kermit now. I wish I had a tape of the muppet's celebrate jim henson. That was the special after his death right?
Edit: I just realized. Jim and Richard's death are probably the reason they are doing this now. They realize how strange the change was and don't want that to happen again so they are phasing alternate performers in now so people get use to it.