Even if Jun is already a route there should be a route for him being hit by the car instead of being a bad ending if he does because seems unfair for the main character to not be able to do tennis because of Jun
Even if we got an ending where we don't just get a game over screen, it'd be a pretty depressing ending. I'd be willing to accept an ending where Jun suffers an injury similar to Yuuichi's, but not if the story just continues from Jun's death