Recycle ideas and plots. If it's a long enough runner then people don't notice. This works best if it's another writer doing it as they'll hopefully make the repeat have a different flavour to the first. Longtime fans will put up with it and newer fans won't notice.
Or maybe most long runners that become bad do it because they are being kept past their use by date because it earns money. So end them.
I'm sure long runners can be good, I just can't think of any good ones that don't rely on negitive continuity or have an ending.