I tend to agree. Bobby's "death" in the cliffhanger episode "Swan Song" peaked at 31.1 HH. When he returned the follow season in the episode "Blast from the past" it peaked at 26.2. The premiere episode the following season(86-87) "Return to Camelot" peaked at 27.1 HH but that occurred in the second half of the two hour premiere. So the people tuning out because it was a dream was hogwash. Dallas started to become stale with the same storylines and as you point out no new fresh characters or storylines. I did like Sue Ellen story with Mandy but everything was just blah. Again, in 1988 JR vs Sue Ellen carried the show because nothing else was interesting. Ray and Jenna together were zzzzzzzzzzzzzz.
1989-1991 didn't have one good storyline. The show was pretty much finished in 1990.
Let me add I did enjoy Miss Ellie story with Wes Parmalee in 86-87. Btw the highest rated episode that season was Bobby and Pam wedding at Southfork.