Being a straight man isn't about being an incredulous naysayer, especially in a comedy narrative. It's about setting up the structure of jokes, driving the momentum of the conversation, being a versatile foil that lets other character's traits shine through, and providing the audience a reference point and the story progresses. And that's what John's character provides in spades. It's one of the few structurally sound elements that Hussie was able to maintain past Act 4 as things spiraled out of control.
And everyone was just kind of there by the end, as Act 6 painstakingly dragged towards the finish line after Game Over, and struggled to find motivation for any of the characters to even care anymore. At least it seems like John's story was at least at some point meant to confront this.