That would mean the old man would be eaten. Shinichi would become an orphan. It seems statistically quite unlikely in any case. Would there even be free parasytes anymore? How long would they survive in the wilds without a host? Some critters or ants would eat them or they would starve.
This ep had a few jolly scenes, like Shinichi waking up without an arm at all and the dad's suggestion for explaining Shinichi yelling to the mom.