5) It’s the first show
It’s an obvious but important fact: Chicago Fire came first. Without Chicago Fire, there would be no One Chicago franchise to speak of. It was the success of this show that allowed NBC to have three other shows. That should count for something on multiple levels.
For one, if you don’t renew Chicago Fire, what do you do with the franchise as a whole? It’s a bit like taking the foundation out of the bottom of a house. Theoretically, Chicago PD and Chicago Med can continue on without it, but it’s going to be a little awkward. Chicago Fire has been part of all but two One Chicago crossovers. The backdoor pilots of both Chicago PD and Chicago Med were episodes of Chicago Fire. By virtue of being the first show on the proverbial block, Fire has also taken on this function of helping to steer the rest of the franchise. What does NBC do if that isn’t there anymore?
Secondly, it deserves some karma points for what it’s made possible. It doesn’t deserve to just be quietly pulled off the air. At a minimum, Chicago Fire deserves to be renewed for a final season and allowed to have some fanfare on its way out the door. It would be an unfair goodbye if it just ended without getting to properly wrap things up. We’re still bitter about the end of the original Law & Order.