The saddest thing about it is that it's nowhere near as sad as it should be. Since the 1990s, the human cast has been increasingly sidelined in favour of gimmicky formats and Muppet material, and young viewers barely have the chance to get to know the humans. Granted, Maria has had more airtime than the other old-time cast members (possibly due to Sonia writing herself into the show more, possibly due to the reality of the ageing cast's availability), but I just don't see how any of the adults would even need a story or explanation for their departure these days.
Sadly, we are long past the era when Will Lee's death led to the greatest, most poignant, most memorable moment in SS history. I can only wonder how may preschoolers even know who Susan and Bob are now. Even though the long-serving cast members have every reason to phase themselves out of the show as they progress in life, are the newer, younger human cast members ever going to have the same significance as the core cast had back in 'the day'?
Yes, I'll be sorry to see Sonia go, but I've been sorrier for a good couple of decades to see the camaraderie from the show's golden era decline.