Oy.
Officially, the stated reasons make sense. At 58, Franklin is probably thinking more about the time he can spend with his family, and I believe most of them live on the East Coast. He's made more money than he'll ever make as a football coach with his increasingly popular Tony Franklin System seminars.
And MTSU sounds like a reasonable place to sail off into the sunset (or wait a year or two before rebooting with a new coach). Franklin had a good year at Middle Tennessee State recovering from his dark few months at Auburn, which eventually led to him joining up with Sonny Dykes in Louisiana Tech.