This is how a lot of TV shows end.
Even the Simpsons made fun of this in an epsiode. Homer states: "We'll be on for years...or until the show is no longer profitable." I guess Fox has hit the financial threshold when it comes to cost per episode vs profit made on advertising. Happened to Dallas, Three's Company, Diff'rent Strokes, and a dozen other shows which were top network brands then declined.
The show was renewed for the next two years. So they have time to put together a quality finale. Greoning could probably propose a new series to take its place on the schedule.
In its heyday back in the 90's The Simpsons was pulling 20 million viewers. Nowadays it barely pulls 6 million viewers on a good week. A final season would be a financial windfall for Fox. They'd probably pull at least 40 million viewers for the finale alone.