Going back to the quote from Maxperson above, I'd say that if what we defeated what we call Evil in a fictional setting, this would establish a new standard for what is Evil out of what was formerly considered Neutral, and likewise the requirements for being called Good would rise, some of what was formerly Good now considered Neutral. To people living under the paradigm of the old "Evil", this would look like everyone is good, but to its contemporaries there would still be perceptible differences separating the new Good from the new Neutral and Evil.
I don't at all agree that this would be true. Having ever-rising standards means the standards are impossible to meet. That's not acceptable. I, as someone who aspires to be as Good as I can be, could never abide by this "welp, you haven't met your Good quota for the Era of the Wombat, it's Inquisition time!"
If you have truly defeated absolutely everything one would call Evil, in such a way that it genuinely can't come back (something I find extremely implausible, but I'll allow it for the sake of argument), then Good says, "Awesome. We can now be focused just on
developing things, we no longer need to worry about putting out fires anymore." By definition, you've already eliminated Evil in a way that it can't come back, so...it can't come back. That would be wonderful in Good's eyes, because it would make a lot of things much, much simpler. It might still maintain a token or minimalist good-protection-force just in case an outside-context-problem arises that reignites wickedness.
See, for example, the Age of Legends in the Wheel of Time books; the Aes Sedai did not become a progressively more and more and more and more draconian secret police force hunting down every possible strain of wickedness, they instead focused on furthering the heights of human achievement. And then, when the Dark One's influence returned, the Aes Sedai became both the greatest defenders of Good...and the greatest
enemies of Good, because the Darklords were all former powerful Aes Sedai themselves (not least because channelers are so much easier to tempt than non-channelers, what with their ability to use the highly addictive True Power that comes from the Dark One himself.) The Aes Sedai had functionally become a body of scholars, diplomats, and civil engineers, with their military training becoming almost totally defunct until it was revived in the period between the unsealing of the Bore and the rise of the Dark One's proxy armies. And Rand, late in the series, even speculates on how the Age of Legends was fundamentally unsustainable anyway; it hadn't so much
eliminated evil as suppressed it under a benevolent but totalitarian regime...and that totalitarianism would eventually be its undoing. In other words, the Good would fall away and be replaced with purely man-made Evil, even if the Dark One was never allowed to leak into the world.
Now, if you want to say that
some settings would work the way you describe--sure! That's a perfectly valid choice, but I think it's specifically valid
because it's the creeping corruption of Law, not Good somehow becoming a bad thing over time. That is, at first, resisting Evil is merely a matter of getting everyone on board with not doing the absolutely awful things, but once everyone no longer does those things,
Law sneaks in and starts whispering "you should
make people behave this way, so they don't stray". And that specific thing is the first seed of Lawful Evil whispering sweet temptations to those who aspire to be Good.