Was the ABC era (1989 - 2003) of movies (https://en.m.wikipedia.org/wiki/List_of_Columbo_episodes) that featured (https://web.archive.org/web/20061031120910/http://www.jumptheshark.com/c/columbo.htm) Peter Falk’s iconic (https://web.archive.org/web/20140403161042/http://forums.televisionwithoutpity.com/topic/3116819-columbo-oh-uh-just-one-more-thing/?view=getnewpost) detective (https://forums.primetimer.com/topic/2189-columbo/#comments) a grave disappointment when compared (https://forums.digitalspy.com/discussion/1615127/columbo-theres-just-one-more-thing) to the 1970s (https://forums.stevehoffman.tv/threads/to-the-fans-of-the-70s-run-of-columbo-ever-noticed-a-deep-change-as-of-season-6.421999/) era (https://forums.somethingawful.com/showthread.php?threadid=3920575)? If so, what went wrong?

No. Both eras of Columbo are great, only episode I have seen and not cared for is the magician one Now You See Him with Jack Cassidy, that one was boring. Rest of the show is top notch though.

Yes, the ABC era was inferior. I think it was due to several factors.
The rising number of cable networks was fragmenting the television audience. This put pressure on the old established networks to cut costs. After paying Peter Falk a generous salary, there wasn’t much money to spend on big name co-stars as there had been in the 1970s. And many of the “classic era” supporting actors had retired or died. There was also less money for top-notch writers.
Peter Falk was named an executive producer, giving him more creative control than he had in the 1970s. And his judgment was not always the best. He would make changes in the writing and direction that would harm each episode. And casting his second wife Shera Danese in many of the episodes was a mistake, because her acting skills were subpar.
Falk himself was aging. In my opinion, he looked older than he actually was. It stretched credibility to think “This Old Man” was still a working detective driving a car that was always one trip from the scrap heap.
That’s the end of my rant for now.