Carell's depature is the reason of the demise of The Office. I haven't watch much of the post-Carell episodes.
The show was declining in quality a couple of years before Carell's departure- after Jim & Pam wed, and the addition of Erin. While there were a few good episodes (wedding, baby birth, and return of Holly for awhile), in my opinion, the episodes afterwards felt inconsistent in quality and there were more unfunny episodes than funny. The loss of Carell hurt even more.
It's NBC's most watched sitcom still. Maybe it's going to take NBC moving The Voice (NBC's top show currently) to 9pm Thu in order for 'The Office' to be put to rest. For one season after Friends, The Apprentice got the 9pm Thu hour.