Spoilers for the S5 finale!
Time travel is complicated, requires a lot of power, and has horrible, horrible side effects.
Unfortunately for Starlight, researching said problems beforehoof is next to impossible.
It really is too bad that she's destroyed the world.
~{^}~
A series of unconnected one-shots that diverge during the Season 5 finale. Since time travel is so arbitrary in canon, I'm coming up with the rules myself and offering what would have happened if it worked that way.