In this what if situation there could be numerous different scenarios.

Perhapes Al Neri and Rocco retaliated, killing Ola and the Rosatos. Pentangeli would probably back them up with more muscle so their side would be superior. Roth would be forced to make a deal with them.

Or, from the perspective that Michael's men where all business men, Rocco and Neri would have sought to make a new deal with Roth, cutting of their support to Pentangeli who would then probably have been outmanoeuvred by Ola and the Rosatos. The other families in New York would most likely have chosen Roth's side as part of their revenge. The Corleone family would have been in the same shape as they were during the Sollozzo crisis, only this time with lesser men in charge.

Or Roth could have manipulated Neri and Rocco so they would go against eachother. Or maybe the subborn Pentangeli would have cut of his support to them because he 'would not have to answer to higher authority anymore'.

In any scenario, there would have been chaos for the Corleone family.