If I remember correctly, and my goodness, I'm basing this off of my memory at 12 years old or so, but it seems like I remember either my mom, or myself?? reading in one of the tabloids that Ken Kercheval's contract was up with season 12, or something to that effect, and we were fortunate that he either agreed, or was asked to stay. Granted, I'm not sure how all the contracts work, and I imagine it is different depending on the respective agents and what not, but I'm also wondering if all of their contracts would have been up at this time, and Ken was just one of many..but I'm not sure how this all would have worked. I do remember reading in the Barbara Curran book that the show was unexpectedly renewed for two seasons during season 12. Unexpected because it was for two seasons, not just one, and the show had slumped a bit in the ratings-nothing terrible at that point, but it certainly was no longer #1, or even top 10 or 20, but I think somewhere around #30-35. Still better than Falcon Crest was doing, and a lot of shows in history have lasted for long durations with those type of ratings, but this was Dallas.
For me, I'm glad Cliff was around for season 13. I liked his camaraderie with Bobby that season, well, when they weren't fighting over the fate of the tanker collision, but that plot was one of the best of those last few years, IMHO.
Now, season 14..I'm glad he was still there considering all that were not, but if I had to choose Sue Ellen or him..I don't know. It might have somehow been better that final year with her, but I don't know if much was going to save it at that point. I have said it before, but I think the show needed a new villain for all the regulars to unite and fight against, or some to secretly be in cahoots with, and no disrespect to George Kennedy, but he either wasn't the right fit as an actor, or he wasn't written well enough to be. Rather than it just being the Bobby and J.R. show at that point, it would have been nice to have Sue Ellen and Cliff till the end, and I didn't really like the turn that Cliff took the final year, but I could also see where Sue Ellen's character had also kind of run it's course.