Michael said before Invincible that that would be his last proper pop album. Obviously things didn't go according to plan with promotion etc. So, it's no surprise that Michael now sees his next release as his last all out assault on the charts.
As someone already mentioned, he will never stop making music though. He's often said how he wants to move more into movies from now on, both infront of and behind the camera. My guess is he'll work more on soundtracks and on-off songs after his next album.
Farewell album doesn't mean no more Michael music. It just means no more pop albums with huge promotion and 5 or 6 singles with videos etc. He's conquered all that. Time to move on to something new. You just know he's gonna go out with a BANG though! :yes: