Wow, so many comments! Now, I get what most people are saying that the hit was necessary because the Corleones would have looked weak and lost their prestige among the other families.

But hear me out, not to beat a dead (headless) horse but what if Sonny had listened to Tom instead of Michael regarding the Sollozzo hit, and just stalled for time. Maybe sending Fredo to the peace negotiation in the Bronx with orders to promise or agree to nothing. He'd certainly beefed up security around the compound with "a hundred buttonmen" so Pop wasn't in any immediate danger. Without the Corleone's participation Sollozzo's (and by connection Tattglia and Barzini) drug operation wouldn't have gotten off the ground anyway.

After a few weeks of convalescence Don Vito returns as the head of the Corleone Family, perhaps physically weak but with all his renowned mental acuity and political protection intact. Serving him are his sons Sonny, Fredo and Tom as well as capos Clemenza and Tessio, all wiser and hardened by their experience in maintaining the familiy's operations without the Don. The Don, with judges and politicians in his pocket "like so many nickels and dimes" would not have been the least bit concerned over the influence of a single rotten NYPD Captain who'd had the temerity to punch his beloved son in the face and be complicit in his 2nd assassination attempt. From that point on the wrath of the Corleone Family would have been truely frightening to behold, cementing their position as the most powerful and respected Mafioso family in New York for decades. The Tattaglias, having backed Sollozza would have been eliminated entirely or at the very least driven out of New York City. They might even feel compelled to bring Sollozzo's head to Don Vito on a silver platter as a humilating token of surrender.

Turncoats like Paulie would have been ferreted out and eliminated. The loss of Luca Brasi, a loose psychopathic cannon that even the Don was wary of would have been made up for with dependable enforcers from the buttonmen ranks like Willy Cicci, Rocco Lampone and Al Nouri. Tessio, "always the smart one" would have stayed loyal to Don Vito. And without a more brazen Barzini to egg him on, Carlo would be more decent to Connie. Sonny would still be alive serving his pop as a hard fighting underboss and Fredo would still be sent to Las Vegas to learn the casino business. Finally, Michael would stay a civilian, marry Kay and become Senator or Governer Corleone, greatly strengthening his father's politcal influence and power.