It probably should have been the last season but no one attached to the show ever said it was. Near the beginning of the season, Carell said he would be leaving and NBC & the producers said that the series would continue without him.
I think the best compliment that I can pay the actors on The Offce (especially Carell) is that they constantly seem like they're doing improv rather than scripted material. I'm sure the reality is that nearly all of what they're doing is scripted but they always make it seem real and off the top...