I disagree with your premise that a "fan" needs to profess blind faith that the team is great, going to make the playoffs/superbowl or whatever. You and others may choose to do so and that's your choice and of course are welcome to it. But others may enjoy candid discussion of the merits (or lack thereof) of their favorite teams players, coaches, office, etc. In my opinion anyway, this does not make them any more or less of a fan of the team.
My hope is that everyone would recognize this and stop trying to tell others how to think, talk, or enjoy their favorite team.