I think many of these decisions were based on Lemay's hubris, rather than just wanting to promote Victoria Wyndham. He wasn't close to Jacqueline Courtney the way he was to Susan Sullivan, Victoria, or Constance Ford, and he and Rauch arrogantly assumed she could be replaced. He also seemingly assumed a number of theater actors he had take big roles on the show would stay long-term. This led to a rot where Victoria was one of the only popular actresses who was staying long-term. And as the canvas was so empty, that made it even easier for P&G to run through dumping based on ageism, like what happened to Beverly Penberthy.
As for Beverlee, wasn't she burnt out of playing Iris the way she had to play her and just burnt out at AW, period?
One of the best parts of Lemay's 1988 return was that he wrote her much more aggressively - there's a fantastic scene which is probably not on Youtube where Rachel icily cuts Liz out of the family after she blurts out to Matt the truth about his conception.