Antonin could've easily been the one to make the first step, to openly declare himself the Dark Lord's replacement. It would've been bloody and many would've been killed or captured, but at least they would've started to make progress with moving on from the "tragedy" that was the loss of their deranged master. The very fact that he hadn't yet declared himself the Dark Lord was just further proof that he meant what he said that day they sat on their sofa in the lounge talking about the future. He had no desire to be the leader. Only his legitimate fears about the safety of his family kept him entertaining the insane idea.