You will inevitably use the screen protector during the lifetime of the device. At some point the device will have outlived its usefulness for performance or other factors. It'll be obsolete to you for some reason and you'll replace it. You'll then remove the screen protector to reveal a perfect screen and it won't matter a bit.
It's not much about preserving a screen until the end of its life, but the ability to swap out a heavily scratched screen protector with a new one 9 months in. If the scratch is directly on the screen you obviously can't do that.