I can't really see what else the writer could do, if Stevens wanted to leave and Mary didn't. Divorces were rare, and for the couple to divorce or live apart would hardly be very happy for Mary...

I think that with Sybil, it would have been better to write her and TOm B out, just have them stay in Ireland....but possibly the Tom actor's contract didn't allow for that. But I think that it is true that killing off 2 major characters in a fairly short time is a bit of a stretch for problability. One is Ok, but two is a lot, esp on top of Lavinia's death. I think that was a reasonable possibility, with the Spanish Flu but for Syb to then die in childbirth, Matt to be killed in an accident...