I must disagree with your opinion a little. CHARLIE'S ANGELS had practically driven itself into the ground by its fifth season. Shelley Hack had been wrongly accused of the ratings decline that fourth season, even though the primary issue with her and her character, Tiffany Welles, was the decision made by the writers to write her into a corner for the first dozen or so episodes.
While Tanya Roberts was attractive, albeit in that cartoonish sort of way, her acting was bad. She's wooden and she hasn't any chemistry with the other Angels, although she and David Doyle seem to gel well enough. I think Doyle was probably more receptive to Roberts coming in, while Cheryl Ladd seemed to think that the writers could've "fixed" Tiffany Welles because it "certainly wasn't all Shelley's fault".
The main issue was that, by that fifth year, CHARLIE'S ANGELS had gotten stale. The show was really past its prime and it was surviving on life support. I won't say that the audience was completely shocked or angry that there was another new Angel, but I don't think it really helped the growing issues, either. That 1980/81 season was riddled with writer strikes, time slot changes, and bad writing. We can all agree that CHARLIE'S ANGELS was never Shakespeare, but that final season just lacked heart. Jaclyn Smith and Cheryl Ladd seemed disconnected and the show itself had quite simply lost its spark. The sizzle was gone, and the staleness took its place.
About the only episode I really like from the final season is "Angel on a Line". That's the one where they are hired to find out who is stalking women over a telephone at a nightclub and then killing them in the parking lot. It was kind of innovative at the time, too. The villain ended up being a cross-dresser. That one was unique. There were a few other episodes that sparked me as interesting, too, now that I think of it. There's one where they're Broadway chorus girls, and those a Western-themed one also, isn't there? Those are decent ones. But that's about it. Creativity was thrown out the door.
Season five was really the worst season. Those opening episodes were drab and very hard for me to get into. I never re-watch that last season. Hardly ever anyway.