The presence of time traveling future rebels in 7/XX implies that even with the hero's efforts throughout the 8 games, the world of Metal Slug is stuck in a Forever War that will last into the far future with no end in sight. The fact that the ending of 7/XX concluded with Morden (once again) escaping seems to further confirm this.
When encountering Mummies and Zombies in 2,3, and 4, if you don't react quickly enough they will infect the fleeing civilians that are in the background, forcing you to perform a Mercy Kill on them. However, when your character gets infected, he/her can return to normal again by picking up a Potion/First-aid kit. Meaning that those poor infected civilians could have easily been cured, yet our heros simply didn't bother to do so and simply killed them to save time.