Isn't it the truth? You make it to Heaven, and then, after a while, it's boring. You can entertain yourself with the thought of how bad the alternative would be, but how long can you keep that up? A million years? You're just getting started!
But unlike the actual afterlife, a play about it is only a couple of hours or so, and you're entirely free to walk out whenever you want.
Bonus music — emphatically not from the aforementioned musical:
"Heaven is a place where nothing ever happens."