There had been a 'war' going on ever since Michael killed Sollozzo & fled to Sicily. As a result of Santino's death the war was stopped and Vito Corleone made a deal. Without Sonny's murder this hitting/hitting back could've gone on for several more months.

As Vito says to Tom in the car, "Tataglia's a pimp. He never could've outfought Santino." Since Vito also figured out from that meeting that it was Barzini all along...looks like Barzini knew what he was doing when he set his sights on getting rid of Sonny.

Anyway, just on the basis of good storytelling...it would be quite unrealistic, quite unbelievable if at least ONE of Vito's children didn't suffer a violent death.

And by the way - Sonny was 'hot' for the Sollozzo deal when it was first presented to Vito. At this point, with his father having survived a shooting and his brother hiding out in Italy...I doubt he was hot for any kind of real 'business' ...which was basically what he told Tom:

T - We can't do business.
S - Well neither can they! Don't worry about it.
T - They don't have our over-head!
S - Please, don't worry about it!
T - We can't afford a stalemate!
S - Well, then, there ain't no more stalemate - I'm gonna end it by killin' that old bastard! I'm
gonna...kill...

T - Yeah, well you're getting a great reputation! - I hope you're enjoying it...

S - Well you just do what I tell you to do! Goddamn it! If I had a wartime consiglieri - a
Sicilian - I wouldn't be in this shape! Pop had Genco - look what I got.

Apple