You have a serious fallacy in your thought there.
If everything generally thought of as Evil is destroyed,
then that does not mean that everything thought of as
Good also would be. The world would just be a nicer
place, and smell better too.

While relative "Good" would be hard to find in
such a world, the lack of Evil would not be something
folks would be laboring and moaning about.

Evil, of course, will never be able to be in the
same situation. Since you attempt to "dominate" and
prove your superiority, there will always be
those opposed to Evil, or downtrodden by it. These
people, in the absence of any other "Good",
would be the opponents of Evil and the heirs to the
mantle of "Good".

While realistically, it is highly unlikely
that either side would ever be truly vanquished,
due in large part to the weaknesses of
mortal minds and frames, theoretically is
much different. Good would be perfectly
happy completely eradicating Evil, but
if I understand the ramblings that often come
to this area properly, Evil would much rather
dominate and subjugate Good, eradiction
is not the goal there.

No stunning revelations involved, but at
least you are thinking!

Covenant Stormcrow

Penned by my hand on the 18th of Scarlatan, in the year 335 AF.