I think Cindy Pickett gave so much inner life to Jackie that no one else could entirely reach. I can see why GL cut their losses with Carrie Mowery, but there was no reason why Jackie couldn't have returned in later years (we know why she, along with Hope and Elizabeth, did not, of course).
Tom O'Rourke was a good actor and a very good looking and charismatic man. I think he could have easily stayed on through the mid-late '80s at least. I have a feeling this exit, similar to Don Stewart's, was down to backstage problems, and similar to Lemay and Rauch's folly in dealing with people they found tiresome, those at P&G, Kobe, etc. figured who needs 'em.
I also wonder if these exits were just to drive home that Philip was a Spaulding, which may have made sense for a time, but robbed him of much of his inner conflict (with just Ross left to remind him of his other side).