Why "The Cosby Show" started going downhill by Season 2

Its only black female writer at the time, Winifred Hervey Stallworth, left for The Golden Girls. And then it arguably went downhill again in Season 3 when Ed. Weinberger left for Amen and Phylicia Rashad got pregnant, killing all the sexual chemistry between the Cliff and Claire. They didn't want to add a new baby like Family Ties did (when Meredith Baxter got pregnant in real life), so they pulled the "hide her behind bags and tables" business.

And then over the course of the rest of the series, it just settled (depending on your point of view or whom you immediately ask) into mediocrity, smugly coasting on its initial laurels that are partly the result of its premiering a year after a particularly dismal year for new sitcoms (though the year before that produced some lasting hits) when all NBC's new fall shows flopped.