Jail time for crimes is primarily about deterring others from doing the same thing. Do you believe we shouldn't be dettering the behavior this guy engaged in?
If we send the message that you can deliberately target opposing staff and players at football games with dangerous projectiles with no meaningful consequences, people will do it more often. Sadly that's human nature.
If on the other hand we send the message that engaging in those activities will land you in jail, fans will think twice before engaging in that behavior.
I'm not of the opinion that this knucklehead should spend years in jail, but I am of the opinion that if he doesn't spend at least a couple of weeks we'll get a lot more of his kind of behavior. That's the main reason we send people to jail.
He'll do his time and he'll be fine. And hopefully we'll have less of his kind of behavior in the future as a result.
PS - I trust he's mostly a good person, but honestly very few 18 year olds in this state would deliberately attempt to cause serious physical injury to another person as he did. That's not what good people do.