I'd say that it would take about 10 minutes for the scabbards to all actually kill Jack, injured as they currently are. Particularly, once Jack chooses to go hybrid, he might not be capable of picking off one of them for their troubles if they rush it. Some people are forgetting Jack's incredible endurance.
By that time, Kanjiro could've easily sent out another suicide bomber to turn the tables and help dispose of them. Inuarashi made a good call when he took on the burden of Jack all by himself.