Like Apple said, Fredo wouldnt have had the same motive to betrey his older brother as he would his younger brother, but sooner or later Fredo's stupidity would have led to Sonny and/or his family in danger. Then I think Sonny's initial reaction would be to beat the ever-loving shit out of him. But Fredo isnt getting any smarter, so it would probably happen again, in which case Fredo would have to go.