You're right Lannis, most of the comedy in those days only lasted for a few episodes and then were junked in favour of new fresh comedy. Fawlty Towers only did a very few episodes but is still funny today.
There hasn't been any good sitcoms for years and the best they come up with keep going forever.
The other problem is they 'over-milk' them for all they're worth using the same lines and bits e.g. that idiot Irish bloke Mrs Brown falls over in every episode. Funny the first time around but after 100 times????
All the good writers we knew are dead now but there must be fresh talent that's untapped. The TV companies just aren't looking for it.