This ALWAYS happens to sitcoms the longer they run. A sitcom's best work is usually done in it's first few seasons, and after that it largely just repeats a formula over and over.
The Office's strength, IMHO, was never Steve Carell (who I found too over the top), but rather in the supporting cast. And as time went on, the show really had nowhere left to go with those characters. Dwight and Jim became friends. Jim and Pam were married. Etc. The show just didn't have a direction anymore and attempts to manufacture one never really 'hit'.
But honestly, can you name a sitcom that stayed strong after 9 seasons? If you can, it's probably only one or two of them.