I rethought this interesting question after Lana reopened the thread:

The most important objective for Barzini and the other Dons was to win the war so they could start doing business again from a stronger position because the Corleones would be in a weaker position. Getting Vito’s police/political protection may have been an important consideration, but the war needed to be won first.

Killing Sonny, the acting Don and street commander, weakened the Corleones and devastated Vito. Although Michael was an unknown factor to them, he showed daring and courage in killing Sol and Mac. He was the obvious next Don Corleone--Vito's last hope. That’s why he—not Appolonia or Don Tommasino—was the target of the car bombing. Fredo wasn’t even a consideration for reasons already stated in this thread. With Sonny and Michael dead, Vito, still weak from his shooting, had no successors and would likely be too physically and emotionally damaged to carry on the war with any effectiveness. He’d have to sue for peace sooner or later.

Barzini and the other Dons could have demanded Vito’s police/political protection as part of a peace treaty with both Sonny and Michael dead. Vito might have refused out of spite or pride, and the war would have continued. But, the Dons would have gambled that he’d settle up, eventually—it would have been too costly, to him and the others he was responsible for, to continue.

Barzini and the others no doubt were pleased when Vito called for a meeting as soon as Sonny was killed. Barzini might thought it prudent to cancel the hit on Michael because Vito had already, in effect, capitulated. But, as others here pointed out, it was too late—when Don Tommasino told Michael that he had to leave subito for Siracusa, he probably meant the next day. And, in any event, Barzini had already ordained Michael’s death as a decisive factor in bringing a greatly weakened Vito to the table.

Certainly Vito would have taken revenge if Michael had been killed after he and the Dons came to terms. But, Michael’s survival actually was a lucky break for the Dons—Vito's overarching, urgent need to bring Michael home safely gave them immediate, total leverage for a quick peace and for obtaining Vito’s protection for drugs on their terms. By their calculation, Michael might or might not have become their nemesis years down the road, but they needed to end the war and get into drugs here and now