Rocco had become expendable by that point.
Why do you think, or more importantly, what made Rocco expendable at that point?
DC, my simple physics is that Rocco was Clemenza's man, Neri was Michael's--just as Hagen was Sonny's choice for brother, not Michael's; and Vito's choice for
consigliere, not Michael's. Michael needed someone to sacrifice himself to eliminate Roth, and he chose Rocco because he needed Neri more.
Others have reasonably asked why Rocco couldn't have arranged for others(s) in his regime to whack Roth--in a plot that might have enabled him or them to get away. And Rocco, alive or dead, would be directly linked to Michael via the FBI chart shown at the Senate hearing. Here I'm
speculating that Michael wanted to send an unambiguous message: Michael Corleone never gives a foe a pass, and will exact revenge no matter how well the foe is protected by the authorities (same reason he wanted Pentangeli's suicide). He made that statement by using Rocco, not others. Sure, in real life that would have made plenty of trouble for Michael. But that's Hollywood.
