I concede some TV today is total trash but the great TV series are concluded when the writer know it is time to stop Fawlty Towers, Porridge et al . Whilst I love LOTSW dearly there are some later episodes which perhaps are a step too far with very thin plots and Still Open All Hours now has a predictability about it because Roy Clarke has clearly run out of ideas and is recycling bits of the Original and far Superior Open All Hours .
If you look at other series held in high[and high-ish ] esteem some sadly have episodes where the plot is really thin and offer little entertainment value , a good example being Dad's Army episode " When did you last see your Money" about Jones misplacing £500 , yet the preceding episode " All is Safely Gathered In " is a classic . To me it marks that the demise of the show is knocking on the door and it is time to consider ceasing production.