I literally already explained why they only need a set quota from Earth, but I'll say it again.
After fulfilling a certain energy quota, they would have extended the lifetime of the universe sufficiently to allow another human-like species to arise, from which they could harvest more energy, repeating the cycle.>>10400>"Inside every cynical person, there is a disappointed idealist."
Now, that's not true for *every* cynic, but it's certainly true for Kyoko.
On the contrary, Mami's philosophy is said to be rare amongst magical girls; most are just in it for the power. Kyoko and Sayaka both became magical girls for the sake of justice, and furthermore, Sayaka is able to connect to Kyoko because of this similarity, and also the fact that she's the only one who can understand Kyoko's feelings of being robbed of their souls.