There was certainly room for both of them to stay, but had they stayed, there probably wouldn't have been any reason for Anne and Claudia in the final seasons. Well, Anne could've still done guest spots, and I suppose Claudia would've been brought on if the producers still wanted to reintroduce Stacy Galina into the show following her short stint as Mary-Frances.
Laura had so much story to be explored. Like it's been said, Richard could've come back to Knots Landing. I like the idea of him coming back to see Laura and his sons before passing, but there's also the possibility that he came back to try and win Laura back from Greg. Laura and Greg didn't need to stay permanently apart if Laura were to stay with the show, but giving them some conflict would've been nice. And that would've prevented Greg and Paige from being a couple in the later seasons.
Abby still had some tricks up sleeve that could've been exposed. I like the thought of Abby falling for a younger guy, but someone who is of good social standing and who actually loves her just as she loves him. It would've been good to see Abby in a romantic relationship that didn't have anything to do with money and power. Whether or not the romance would've been retained for the duration could've been a toss-up. Had the romance ended, Abby's leftovers could've gone to Paige, if the producers chose to retain the actor.
KNOTS LANDING could've used some of that heyday stability in the final seasons outside of Karen, Mack, Gary, and Val.
Oh, and while we're at it, I would've rather Lilimae come back instead of Aunt Ginny popping up out of nowhere.