I don't buy that the original writers of The Simpsons didn't like having sentimentality in their scripts. Having listened to commentaries for most episodes up through season 17 it sounds like it all depends on the writer. Some lean sentimental, others don't. The staff sounded very proud of episodes like Marge Be Not Proud and Bart Sells His Soul.
Now I would say that show runners play a part in the tone of the show, and Mike Scully was notorious for off the wall humor and more mean spirited characterizations. If anything, it was Al Jean that brought sentimentality back to the series when he took over in season 13. The Critic, which Jean ran before returning to The Simpsons was almost too sentimental and schmaltzy at times. Recent seasons have had some real good heartfelt stories, and I think the staff actively try and pull at the heartstrings and go sentimental. They don't always succeed, but recent episodes like I Won't Be Home For Christmas, To Cur With Love, Holidays of Future Past, Halloween of Horror and How I Wet Your Mother went the emotional route and were effective. Holidays of Future Past was particularly poignant when adult Bart and Lisa reconnected while drunk in the treehouse. It was meant to be the series finale, and a part of me wished it was or that they shelved the episode for a finale because it was so good.
