My take, even if Sonny was not killed, it was never going to work for them - Well, Buckleys and None - known murderous, top Mafia, Corleone family and Michael Corleone in "this clean country, in his oily hair -- dressed up in those silk suits and try to pass himself off as decent American Senator"!
I too reckon, Crime and Politics never going to work for them but Crime and "legitimacy" could, same as Pentangeli and Michael
