Fair point. Not many series' can maintain the same high quality & probably no coincidence, that the last series of any run is often the poorest, as that is probably the reason they called it a day. And the reason why any series can only keep it up for so long before the quality wanes & eventually, if the creators themselves haven't realised, it eventually gets cancelled by the TV producers anyway.
I think the only exception to that rule seams to be, Last of the Summer Wine - Series 23 & 24 [DVD]
God only knows why that ran for so long.