I probably might get razzed for this viewpoint, but I think there were really four stages of when it jumped the shark, in this order...
1. Timeslot Musical Chairs in Season 5
2. Move to California after Season 5
3. Betty Garrett (Edna) leaving at the end of Season 6
4. Cindy Williams (Shirley) leaving early in Season 8
Granted, I don't think it should have gone to Stage 4 and wiser minds should have let Season 7 be the final season.