The novel kinda took on that possibility. Sonny suggested that someone up and coming in the family negotiate with Sol instead of Michael, but Clem said it'd be like sending a rookie to pitch in the World Series. "It has to be Mike," Sonny concluded. Also, the novel notes that Sol had a man in the restaurant watching out for treachery. When Michael asked to go to the john, Sol looked over to the guy, who nodded his ok, as if to say he'd checked things out. I think that guy would have spotted the Corleone men before Sol and Mac entered the restaurant with Michael. Sol would have been looking for that kind of move.