Joab means if David did not go and speak kindly to his soldiers, they would all leave David. This can be stated positivley. Alternate translation: "only if you go will any of your men remain with you" or "if you do not go, all of your men will leave you"
This refers to them remaining loyal to him. Alternate translation: "not one man will remain loyal to you" (See: [[rc://en/ta/man/translate/figs-idiom]])