The reason that Welcome Back Kotter lasted four seasons was because of the executive producer James Komack. He ran a very toxic environment especially to Gabe Kaplan and Marcia Strassman. Both Gabe and Marcia both wanted an at home episodes but was thwarted by Komack. Then Komack got rid of Gabe's creative partner Alan Sachs and brought in inferior writers. Gabe left angrily and the fourth season was horrible.
Review of Welcome Back, Kotter
