There is really no answer to that because it wasn't given a chance to happen. If I posessed the mind of a mafia kingpin out to eliminate my rivals, I might have some kind of plan for you.

But let me ask you this, since you want to get into 'specifics': Do YOU have a way to guarantee that even 'from exile' Fredo would definitely NOT one day posed a threat to Michael and/or the Family? Either on his own or via some rival plot?

Especially after that 'stepped-over' rant in the lakehouse?

Oh, and for what it's worth, what a riveting, bone-chilling conclusion that would've been to GFII. Fredo being put in a limo and driven out of town. Far more effective than a Hail Mary and gunshot.