I can't agree with you there. John Cleese and Prunella Scales were so clearly the "backbone" of the series, remove them and and there would be nothing left.
I think you have to find series where one of the supporting actors grows in stature to completely outshine the original "stars". Someone mentioned The Fonze in Happy Days..an excellent example.
Cheers was interesting, as someone posted, it was a series that went through a number of changes.Sam and Diane were obviously meant to be the main characters, but their storylines became tiresome. and It was apparent that most people were watching for the inter-action between the supporting characters..Carla, Coach (Woody) Cliff and Norm etc.
The introduction of the superb Kirstie Alley served to "save" Ted Danson's character who was becoming a predictable bore, and changed the dynamics again for a while. Then along came Frasier making it more of an ensemble piece.