If it had not been the toll-booth, Sonny would have been killed some other way. The whole point of Sonny’s character was to show what happened to a mobster who let emotion rule the day. In part II and Rothstein was explaining about Moe Green’s death, he kept saying that he knew that it was only business.
If you got emotional as a mobster, then you would never become an old mobster, as Sonny did not.
In part I when the Don and Michael are in the garden talking about the other families moving against Michael, the Don avowed as to how he knew that Santino would be a bad Don. He was unable to separate the business from the emotional.
He knew Sonny was not fit because he opened his mouth too much, like when he opened up his mouth during the original meeting with the Turk ("Never tell anyone outside the Family what you're thinking again."). He unknowingly exposed Don Corleone, because the Turk figured with Vito out of the way, Sonny would be willing to make the deal. Vito probably never forgave Sonny for that.