Maybe your logic is good but Sonic's food quality and menu defies logic. Franchise owners couldn't give a hoot about quality, customer service, or just being plain good. Corporate seems to lost their way and/or have no influence with franchisees. Being raised in "Sonic" land and eaten there until a few years ago I feel well qualified to opine on the menu, quality and tastiness of its offerings.
Having said all that I'm sure smart folks will think they can change things. Good Luck.