What I mean is that he went from (https://web.archive.org/web/20061031123517/http://www.jumptheshark.com/a/allinthefamily.htm) being a bigot who was was wrong about everything to merely a grouchy (somewhat even liberal) curmudgeon who was mostly right.

I think it stopped being as funny for the same reason other successful sitcoms stop being funny. The successful characters become mutated forms of their original selves. The writers just write lines to get funny laughs instead of writing scripts that develop the character. Golden Girls suffered the same fate. Threes Company too. The list goes on. All the scripts are written for one liners to showcase the outlandish behavior of the character on the show. Instead of Archie showing his bigotry in scenes in the script, the script now writes Archie an angry white guy spewing bigotry line after line for quick laughs.
The episode where Joey is born highlights this. Archie is in blackface at the hospital. Its just over the top silliness for a laugh line.