I think it’s possible for a non-native speaker to achieve perfection in writing or let’s say near perfection, not to mention the possibility of being better than the vast majority of native speakers. For serious writing, both native and non native would employ an editor anyway. I’m a non-native speaker by the way... I struggle with a couple of things (like articles! - wrote about it recently) but I think it’s possible, in theory, to improve and be perfect