Oli, my analysis is that Michael was holding onto NY for three reasons: First, he got tribute from Pentangeli. Second, some of the NY high rollers were potential customers for his Nevada junkets. Third and most important: NY was Michael's muscle-in-reserve: a deterrent to other gangsters possibly wanting to horn in on his holdings. If Michael had been killed in the Tahoe shooting, Pentangeli would have been blamed. He'd have been on the run, and the Rosatos would have stepped into the void and presented Michael's successors in Nevada with a fait accompli. When the Tahoe shooting failed, Roth went to Plan B: Lure Michael to Havana, get the $2 million from him, and have him assassinated. Michael had already told Roth, in Miami, that "Frank Pentangeli is a dead man." Even though he didn't mean it, it cleared the way for the Rosatos to whack Pentangeli. Since Michael was supposed to be killed in Havana, it would be the same fait accompli for his successors in Nevada.
So, what would happen in Nevada if Michael had been killed?
As you point out, other Dons have been killed and been succeeded successfully. But, Michael's Nevada operation wasn't a Mafia family, strictly speaking: it was semi-legitimate, and regulated by the Gaming Commission. If Michael died, his political and business connections that kept the enterprise treading the narrow line between legitimacy and criminality would die with him. Fredo would be the nominal heir and successor. But, he was weak and stupid, and fatally compromised by his participation in the Tahoe plot. Even if he didn't know it was gonna be a hit, his coglionii would be in Roth's pocket forever.
Rocco and Neri were forceful and ruthless, but probably in the dark politically. Tom would have had some of Michael's political/business knowledge and contacts. But, he'd probably support Fredo, which would put him in possible deadly conflict with Rocco and Neri. All Roth would have to do would be to sit back and let them fight each other. Then he could go to the Gaming Commission and convince them that important Nevada sources of revenue--the Corleone casino holdings--were in jeopardy due to an internal war among gangsters that was blighting the reputation of the gaming industry. The Gaming Commission would step in, seize the licenses and put the casinos into trusteeship. Guess who'd pick them up after that?
