Kaido, Whitebeard, and Sengoku all actually do things though, and protect their subordinates. Dragon let most of his top brass get swept away without much fuss and wrote them off as lost causes quickly. His strategy makes him especially replaceable actually, since his role requires little personal strength. Anyone with the brains could do his job, few can do Kaidos/Whitebeards/Sengokus.
They'd do the same they had been doing until then, gathering forces and training new members. In fact, one of the new members (Sabo) was the one to declare ware on Dragons behalf.