#aters

It's a metaphor! The entirety of Sad Face was a metaphor for Adventure Time!  The circus is Adventure Time!  The chipmunk is the plot!  The boss bug is censors/Cartoon Network/head execs/etc! Blue Nose the clown is the concept of creativity!  The bug audience is... the general Adventure Time audience! 

Blue Nose wanted to have a more melancholic show. Make it avant-garde with real emotion. The audience was not entertained by this, so the boss bug had to keep the chipmunk locked up. Blue Nose saw that the chipmunk wasn't a terrifying monster, and it deserved to be free in the wild! But as long as he continued trying to do his own thing, that didn't entertain the audience as much, the chipmunk had to be kept under lock and key by the boss bug.  Eventually Blue Nose had to go back to silly, entertaining things to allow the chipmunk to run free. The boss was angry Blue Nose allowed the chipmunk to escape, but in the end Blue Nose escaped and will, more than likely, return next month as always.

SEE THE METAPHOR? 

Adventure Time is trying to be more serious. Have real story-arcs with lasting consequences. To do this they need to cut back a bit on the humor. Not all the way, just enough to allow a bit of tragedy to stretch out past the comedy.  Problem is, not everyone likes that. Maybe we teens and young adults love the plot that builds around Adventure Time's lore, but younger kids (Or at least the Cartoon Network execs who supposedly research younger kids) won't be able to follow the idea of a "silly" show like AT having real depth... And the adults who watch the show with their kids often don't want their kids to have to deal with shows with such substance.  To entertain the masses, AT's plot has to often be kept toned down, away from its full potential. "Kids" (again, what execs think kids are) might not understand the deeper meaning of an episode like Too Old. They WILL be happy that, "yay! popular, colorful characters that I know and love such as Finn the Human™, Princess Bubblegum™, and the Earl of Lemongrab™ are here! I am entertained!"  If things start changing... Well, the audience won't be entertained. That's why things always return to status quo. "Why has Jake stopped adventuring as often with Finn? He has fatherly duties to take care of? But that is not entertaining!"  So Adventure Time has to choose between an episodic saga with little humor that loses views, and an episodic comedy with occasional serious story arcs that the execs won't nix. By airing Sad Face, it's obvious which they've had to choose.