I think Solozzo would have been looking for a hit man in that restaurant--he probably had a sixth sense for that. The noveol says he had a man at another table in the restaurant. When Michael asked to go to the john, Sol looked toward the guy for an ok.

But, I agree that Michael didn't have to be the hit man. Michael told Sonny and Tom, after he volunteered to kill Sol and McCluskey, that the Corleones could get away with killing a police captain because he was "mixed up with murder and drugs...We have newspaper people on our payroll, Tom--they might like a story like that." Carry that a step farther: Tom could have leaked that story to the newspapers without Sol and Mac being killed. It would a major scandal that the papers would have given it such a big sendoff that the Commissioner would have had to suspend Mac, and put his own men to guard Vito. Sol would have been hunted down by police and be killed "trying to escape" before he'd be able to blab about his relationship with McCluskey. And, Michael could have gone on to college. And. we would have had no Godfather Trilogy.