This was my first thoughts when I saw this thread. Andy should have just dumped the show after Knotts left it. Not nearly as good.Not really the same show.
MASH went on about three seasons too long. It got incredibly silly the last couple of seasons with about three or four good shows in that time. They should have dropped the show after Larry Linville (Frank Burns) quit. Just not nearly as good without him. I never thought the Charles Winchester character was not a good fit.
Really most shows that go past five seasons start to really grasp for ideas and probably should have been dropped.