https://www.rollingstone.com/tv-movies/tv-movie-lists/worst-tv-network-decisions-1234796350/the-ropers-quit-threes-company-for-ill-fated-spinoff-1234796671/

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The Ropers Quit ‘Three’s Company’ For Ill-Fated Spinoff

Many of the most successful sitcoms of the Seventies were spinoffs, including The Jeffersons, Maude, Good Times, Rhoda, and Laverne & Shirley. And when Three’s Company became an enormous hit for ABC, the producers floated the idea of creating a new show around the nosy landlords the Ropers. This meant removing actors Norman Fell and Audra Lindley from a guaranteed hit (and payday) for a show that might fail and leave them unemployed. To quell those concerns, Fell and Lindley were promised they could return to Three’s Company if their new show, The Ropers, was canceled in less than a year. In the meantime, Don Knotts was brought on as the new landlord on Three’s Company. The Ropers hung around for a season and a half despite dismal ratings. By the time the network yanked it, the one-year mark had passed and Fell and Lindley weren’t invited back to Three’s Company. Fell later said he felt ABC purposely kept The Ropers on the air just long enough to make sure it didn’t need to bring them back. Whatever the truth is, it’s a good life lesson: If you have a great thing going, don’t risk it by looking for something even better.

Technically, Norman Fell and Audra Lindley were invited back to Three's Company following the end of the Ropers spin-off. They appeared one last time in the Season 5 episode, "Night of the Ropers" (https://threescompany.fandom.com/wiki/Night_of_the_Ropers).

Pertaining to Three's Company, I would argue that Suzanne Somers' conduct during the aforementioned fifth season was a bigger bad decision.

I would also argue that Three's a Crowd was probably the biggest screw up of all because the final few episodes of Three's Company basically function as a glorified backdoor pilot for Three's a Crowd. At least the Ropers leaving for their own show didn't ruin the show's legacy or tenor overall and put a sour taste in viewers' mouths not unlike Three's a Crowd.

Threes Company rocketed to the top of the ratings right away. There was talk of a spin off during Season 2 but they declined the offer. When the show was such a hit, the Ropers were instrumental parts of the show and why it was successful. Mr Roper was a frequent foil for the kids. The banter between Mr and Mrs Roper was hysterical. Many times Threes Company would actually begin in the Roper's apartment for some funny banter before moving upstairs. They were so funny the network thought they could carry their own show. However they were like a fantastic dessert or a fine spice in a great meal. They are what made it more special. That banter could not carry an entire sitcom. Ironically the Ropers fell flat because that sexual tension was rarely seen on The Ropers. Mr Roper had nothing to do and they tried to make his tension with the neighbor the conflict like he had with the kids. However it never really worked. Stanley had power over the kids as the landlord and would barge in their apartment all the time. These neighbors just did not like each other.
Just imagine if Stanley was the maintenance worker for the townhouse complex. At least it would have given him a purpose to annoy people. Helen also suffered because she was not around the young kids for that sexual banter.
Long story short, people tuned in to see the Threes Company Ropers but it was not the same dynamic at all.

They should have waited until the show was over to spin them off, like they waited with “George and Mildred” from “Man About the House.”