Forbes speculates thusly:
... Though Fox has made billions from The Simpsons since the show debuted in 1989 from ad revenue, video games, movie tickets, and theme park rides among other things, the cash has dried up in recent years as viewership plummeted. Odds are slim that The Simpsons will ever be able to recapture the ground that it has lost though it continues to have legions of devoted fans. ...
Aren't the devoted fans the point?
Kantar Media's educated guess is that The Simpsons pulled in $94 million in ad revenue in '17, and the money flow is declining. But of course there are merchandise sales and ancillary rights, and the thirty-year-old cow still produces hundreds of millions of cash.
And of course there is one large rodent in the room: Disney didn't buy Fox so that the entertainment conglomerate could turn around and shutter the franchise. If a new Simpsons feature is actually in development, why not two or three or four? The Walt Disney Company has a history of squeezing acquired franchises until every last drop has leaked out; witness the Intellectual Property of Lucasfilm (purchased by Diz Co. for billions) as exhibit "A".
The Disney purchase should give pause to anyone who thinks The Simpsons will soon fade away. They might not keep making a half-hour sitcom, but there will be specials, there will be streamed shorts, and there will be features.