Dallas failed in the final seasons because it turned away from what has made it so great. I've said a million times that all shows have a shelf life, and nighttime soaps were on the downswing all around. Dallas tried to change with the times, tried to appeal to viewers who were turning off soaps in general, but in doing so, it completely destroyed itself.
Season ten was "fine." But not great. The Wes story was fantastic, but it had no real resolution and they just dropped the whole thing. Donna was removed from the family and shoved into a romantic relationship with a DC senator that no one cares about. Ray and Jenna were forced together in the most unbelievable and annoying "romance" in TV history. Bobby and Pam sort of hovered in the background, with Pam's only real scenes being shrieking about Jenna or JR. Ellie and Clayton had about as much to do as Teresa. Sue Ellen had a great first half of the season but then turned into a scared and insecure little mouse again when Mandy came back on the scene, only to snap back into bitch mode with no explanation or warning to fire her. And JR... this season was the beginning of the end of the JR mystique. The Calhoun story line as well as the losing of Ewing Oil were both interesting, but made him look weak and timid.
By the time season 11 rolled around, there was no Pam, no Donna, and Clayton and Miss Ellie further marginalized. They wedged in Nicholas and Kimberly and April, and none of us cared about any of those people. Jenna was shoved even further down our throats,and the characters were all behaving in ways that weren't true to who the audience knew them to be . The most egregious mistake was the absolute discarding of Pam - who had been a central force in the show - to the point no one seemed to give even the slightest damn what happened to her.
Seasons 12, 13, & 14 were nothing more than them bring ing n young characters - and in some cases (specifically James) - using "pretty" actors who couldn't even...well, act. One bleached blonde bimbo after another to try to steal Ewing Oil, and JR and and Bobby running around acting half their age. It was a mess.
Is Katzman to blame for all of that? Well, some of it for sure. But I suspect the real culprits were Duffy and Hagman and their egos.