I agree with all of the above. Plus, Frankie was too obviously set up as the fall guy for the shooting. What would Frankie have to gain from whacking Michael? To run his family without Michael on his back? He'd never get away with it. Frankie was, as Roth later said, "small potatoes."

I think Michael made a near-instantaneous calculation right after the shooting, and figured that Roth had far, far more to gain from his death. I think he saw very clearly in that moment that Roth resented Michael's relentless incursions on Roth's Nevada gaming empire and his designs on Havana. What's more, Roth was "legitimate"--where Michael wanted to be--and would continue to be legitimate after Michael was gone. But Frankie would still be a Mafia guy, with neither the "wit" (Michael's word about Zasa) or the imagination to achieve more.