Not surprising that the ratings have fallen. There have been some good ones in the Comedy Central run, but on the average it hasn't been as good as the original series. Better for the show to go out while it still has some quality in it rather than limping along tiredly well after it should've ended, like The Simpsons
“Perhaps Futurama will return in another form, on the Internet, or as a puppet show in the park,” says Groening. “Or maybe as a puppet show in the park on the Internet.”
I bet some fan out there is alread planning to make a YouTube video based on this...
, since this was actually one production season that's been split over two broadcast years, the finale was surely made quite some time ago.