You seem to be just fine with re-casts of core characters in primetime shows (like daytime soaps do all the time). A lot of us don't think that usually works very well.
Yes, the lousy writing from the closing years causes everything else to make little difference. Phyllis Diller could have played Pam by then and no one would have noticed because no one was watching. (From what I hear, Diller turned them down).
Obviously, if an actor (like Jim Davis) dies, or an actor (like Patrick Duffy) leaves and the character dies, it's incumbent upon the producers to write the character out effectively -- to make their absence a character in itself (at least for a while).
They didn't do that with Pam. They just didn't seem to want to deal with it (especially given the harsh criticism they'd received for the cheapy way they'd executed Bobby's de-execution). Sure, later on, they did the Jeanne O'Brien storyline with Margaret Michaels -- and that really had potential -- but then that sputtered out and went nowhere (other than giving Bobby a chance to say, "Goodbye, Pam" in a freeze frame outside Jeanne's apartment -- was it snowing or have I imagined that??) because Lenny & Lakin just didn't seem all that interested in it.
In "Carousel" in 1988 (also with Michaels, and a scene I like) they seemed to say outright that Pam was terminal. And that was about it. The DALLAS brass still seemed disinterested in it.
But that brief moment, although a re-cast, is so shadowed and ambiguous, it almost feels acceptable. It's not "too much" and we're not entirely sure whether we should take it seriously.
Clearly, the show indeed had already peaked. And, as they say, it's Act Three which really determines the quality of your product --- and we sadly know what happened with DALLAS in that capacity.... I have no problem with Pam leaving when she did, as mysteriously as she did, because nothing lasts forever -- but
just frickin' write it, forgodsake!
It wasn't until nuDALLAS addressed it a quarter century later (one of their best plots, bytheway) in late-Season 2, that we got any details about it at all!
Sun in Capricorn/Aquarius Rising -- before all the surgery, they had the same face, the same lake-dunking in their respective pilot episodes, the same truck-collision exits, and the same slapping skills.