Community is a great show. I don't know if you fellow Human Beings have realized this, but the new plot introduced allows them to have a limit of only 4 seasons.
Did you see the most recent episode? They hung a pretty big lampshade on this (roughly paraphrasing from memory)...
Psychologist: ...Don't Community college degrees only take two years to obtain anyway? Why would you still be attending?
Jeff: I'm tired of everyone asking this! Community college student attend their school of choice for an average of five to seven years!
So yeah, if they wanted to keep the show going (for six seasons and a movie, let's say), the writers have a justification.