I think Carlo tried to make Tommy a successor at one point. Joe Bonanno tried to make his son a boss after he passed on too. Obviously neither worked. I suspect that the general opinion of them would’ve been different had they killed someone for the family. Maybe then they would’ve had enough backers to take the reigns. Gambino wanted a successor to take the family to the next level with legitimate business. That’s why Castellano was the ideal choice for him.