even mid-tier stuff like marvel's x-men and spider-man shows are pretty good and those have long seasons. Family Ties has a couple good seasons and then it falls off a cliff but that's not because of the season length the show just gets bad.

i can't think of any shows where like 1/4 of the episodes are good and the rest are bad, those just get written off as bad shows overall and not making half the episodes wouldn't have saved them. maybe dragonball Z.