I agree, Sonny and Michael would have found a way of operating together, perhaps Sonny as a wartime consigliere
But with the Sollozzo story behind them, outsiders would know how to crack the bond between the two brothers. Greed and hot temper were Sonny's weaknesses. A new Sollozzo or Roth might have used that information. Or he might use the knowledge that Michael knew that those were Sonny's weaknesses. So he could have put a seed of mistrust in Michael's heart.