I think in a discussion like this people tend to forget how rethless and brutal the mafia has been in the past. The only reason they seem softer now is because the fbi is constantly on their ass. The pressure is so high that its just bad for buisness to whack people who cross them. If you take that out of the equation the gangs would have a fight on their hands. Plus as other people noted, if gangs started dropping bodies of mobsters all over new york, LE would crack down on them hard.