The High Point: Episode #1 to #75 (Jackie Becomes A Trucker)

Trending Downward: Episode #76 (Darlene Fades to Black)

Downfall: Season 5

I was never a fan of David, Nancy or Fred. They had very little charisma. The show started to have a "white trash feel" with their inclusion.

Another reason for the downfall was when the show started to focus more on Darlene and less on Becky. Granted that probably had something to do with Goranson's scheduling issues and impending exit. Even today the writers are making a mistake by not focusing "The Conners" around Goranson.

Generally, I loved the show a bit longer than you, and would push its downfall at least a season later, 6 if not 7 (don't remember really well).
I greatly enjoyed it with Darlene becoming pessimist, disillusioned etc, and the David story-arc, but of course the series' best season were until the 4th.

It is a pity for the show that Goranson had to leave for her college schedule.

But I really agree on your last phrase: today on the Conners the writers are making a huge mistake by not focusing on Becky.
The Conners started good, really good, and now is a bit of a mess. Becky is the only one that can save it.