It's actually a really reasonable reaction when considering that the villain in question is capable of stealing magic. It's easier to protect one source than four, not to mention that four separate power sources won't be able to accomplish as much as all their powers combined.
However, I doubt this is permanent, because in the end, Twi's still gotta hang with her friends, and she can't do that when she's taking care of the sun, AND the moon, AND the Crystal Empire.