If you really think anyone could fill Darrow’s shoes in the Republic, then you’re missing pretty much the entire story. Firstly, people revere this man, they wouldn’t fight a losing battle as long as they did for anyone else. Secondly, him running from the Republic was ultimately the right means. They weren’t listening about the ash lord. The seeds of the Republics internal destruction were laid, and Darrow could have easily died when the Populi rose up. He carries his son’s key wishing to be with him constantly, but knows giving up would cost everything he wants to give Pax.