In the case in which humanity is modern enough to scientifically reduce a planet into a battery, you'd think they'd also be advanced enough to be able to hop planets.
Planet hoping is a terrible idea anyway. There's not even a planet close and good enough for our mankind to go to, even mars would not even be good enough replacement for earth in 4000 years.
That said, their humanity was the same as our current world. Potimas only figured out about
mana, because angels and dragons existed on their planet.
You'd think that a evil Goddess that pretended to be a highschool gamer girl would have little more common sense too.
D thought "Wow, hooman are stupid, they ruined the planet then forced a good Goddess to take the blame and take her mana. Lol, let's force them into killing each other to farm mana to save the planet."
So D gave them the system- where living beings can be broken down as Exp. The more they kill each other, the more mana they have. Hence when they die, the planet is healed with mana.