Contraries are beings presupposing each other in sense, i.e. beings whose meaningfulness is conditional upon that of their opposites.
Meaningfulness is to understanding as possibility to existence: i.e. meaningfulness to an understanding is nothing other than real possibility, described with respect to an understanding rather than reality as such. Those who think otherwise ground meaning and possibility alike in their own ignorance.
Hence, contraries are beings whose possibility is conditional upon that of their opposites; as Aristotle says, potencies are directed at contraries.
So for goodness to be possible is for evil to be as well.
A world where goodness is possible is better than one where it isn’t.