Times of Change
Okay, most people say 'I'll be Waving As You Drive Away' is the last point of purity in LHOTP and 'As Long as We're Together' (Season 5) is the beginning of the end. I agree there's a noticeable dip in quality at this time and you all make a good case for it.
But I think "Times of Change" (Season 4) is the real beginning of the end and here's why.
I grew up with the first three seasons and loved them. The acting, screenplays (for the most part) music, locations and characters are excellent. I especially loved the Edwards family because they served as the perfect complement to the Ingalls and each of them had their own distinct personality--rough, gruff, brilliant and hilarious Mr. Edwards, prim and proper Mrs. Edwards, wistful, melancholy, poetic John Jr., feisty, mischievous Tom-Sawyerish Carl and dear sweet little Alicia. I never saw further than Season 3 so when I finally got around to 4 as an adult, I was curious as to how the show would develop them later on.
Suddenly--POOF! They're gone! One measly line thrown in the script (Times of Change) that they left for California. There's no explanation, no episode to show them leaving and the Ingalls helping them pack and giving them a farewell and Mary goes to Chicago to find John Jr. has dumped her for a rich bitch, Willoughby style. (Sense and Sensibility/1995 film)
I couldn't believe John would ever forsake Mary because:
1. He showed the willingness to sacrifice his scholarship for her.
2. He was building a house and plowing a field to prove his worthiness.
3. He struggled with low self-esteem and feelings of unworthiness because of the bear incident. So he was eager to please Mr. and Mrs. Edwards so they wouldn't think they had made a mistake adopting him.
4. He didn't want to leave Walnut Grove at first because he was afraid, he might lose Mary.
5. He was always respectful to the Ingalls and Edwards. So why would he burn his bridges with the people he owed so much to--who helped him find the right parents when his mother died so he and his siblings wouldn't be separated?
Plus, the girl he was with in Times of Change was bitchy to him in every scene. Mary gives him a homespun shirt as a present and we're supposed to think he would still ditch her for a girl with an attitude like Nellie?
It's like Michael Landon and the Raschella couple (screenwriters) thought the way to tie him up was to make the audience hate his guts.
The corruption of the Grange lobby subplot was pointless too--just another chance for Pa to show off how noble he is as if we needed more emphasis on that. (More self-aggrandizement on the part of Landon.)
Also, Victor French was such a vibrant presence in the show so when he left, there was a void that couldn't have been replaced. Can you imagine Andy Griffith without Barney or Fraiser without Roz, or Touched by an Angel without Tess?
I know "Times of Change" was the product of internal management conflicts, but I couldn't believe they couldn't find a better way to write an exit story for the Edwards family than the way they did.