This is a comedy show and not some existentialist drama about life and bad things that can happen.
No. It's a show that the writers want it to be. Just because it's cloaked in the trappings of a comedy it doesn't mean that it should limit the writers from telling the story they want to tell.
I repeat myself.. it is a comedy show and it should end funny.
Was the M*A*S*H
finale funny? Was The Wonder Years
finale funny? I seem to remember both had plenty of moments where it was heartfelt and dramatic. There's no reason this sitcom can't end similarly, if that's what the writers want to do.
Whether we like it or not is entirely up to us. But how it ends is entirely up to them.