The entire assertion of this post is erroneous.
Dallas was not in decline yet.
Overall Ratings were #2 and 20.97 ratings points.
The previous season was #1 and 21.5 ratings points.
A "decline" of 1 place and barely one quarter of one percent in ratings points.
Any show would have traded spots with Dallas, except for the #1 show, obviously.
Added to the cast was dack Rambo (a seasoned actor), Jenilee Harrison (surprisingly strong performance) and
Deborah Shelton (whose role would be significant for JR, Sue Ellen and Cliff)
Morgan Brittany would play a significant role in events, and William Smithers returned.
And I do think Donna Reed brought some positives to the role, including dressing more fashionably as a wealthy
Texas woman
Decline began in the next season, the Dream Season (Season 9)
Ratings at #6 and 18.8 ratings points, a 10% drop even though BBG returned.
Season 10: Ratings at #11 and 18.6 ratings points, probably would have been worse without Patrick Duffy's return
Obviously, recasting Miss ellie had no detrimental effect on ratings, and BBG's return had no positive effect on ratings.
Yes, BBG was iconic as Miss Ellie, but people didn't watch Dallas to see
what Miss Ellie was doing.
Ironically, there was probably more interest initially to see Donna Reed in the role.
People watched the show for JR, Sue Ellen, Bobby and Pam.
and to some extent, how Cliff would try to defeat Ewing Oil.
Donna , Ray and Miss Ellie were basically good supporting characters.
Again, it was not a season of decline
en.wikipedia.org