Well, BofB, "what-if" is always the issue in organized crime--the whole damn business is so risky! I guess that Lansky and Costello--and especially Luciano, who brought Genovese along--knew their man. They guessed, correctly, that Genovese was too ambitious, too greedy, to pass up the deal offered by Nelson Cantellops that they had set up. Another factor that I'm sure you read about: Genovese had killed his wife's first husband so he could marry her. But, according to some sources, she was tough, independent (and bisexual, according to some accounts), and she talked against him with impunity. Some say she helped send him away.
This gets to a theme that's repeated constantly in this particular board: being a Don, which we tend to romanticize here, is actually just about the riskiest thing a criminal can aspire to. There's no honor among thieves. And the higher you go, the more people are gunning for you--and the harder you fall. A major subtheme in the GF Trilogy is how the top guys, including Michael and Vito, could never trust anyone, could never be free of doubts, even about those closest to them.