As far as I know, Matt only stays on in a producing/advisory role. James L. Brooks is the only creator that still stays on in a full-time capacity, if I remember correctly. Al Jean and those guys are the driving force behind the show (again, if I remember correctly).
Point is, if Matt and James wanted to cancel the show, they'd be able to do so, or at least draw the fans away enough that viewership would drop.
I suppose so, but this is also a money-making thing for him so I can kind of understand why he wouldn't just pull it. I also don't feel like viewer counts of the Simpsons has really gone down. I think a couple years back it had gone down, but now with the recent years it's picked up again. At least, it feels like that to me.