If you are really diligent with your retainers for, like, the rest of your life, you should be able to maintain that perfect grill. But stop wearing them, and your teeth will settle into their own character. I had braces when I was an adult, and before, you could tell that I never had braces; one of my front teeth 'hid' behind the other. Got braces (traditional, metal mouth) and 23 months later, the treatment was done, and I was sent home with my retainers. I was good with them for the first year or two, then got lazy, and I don't even know where they are anymore. My teeth are not clinically straight, they are their own people, and I'm perfectly happy with the result.
As most others say, I should have done it long before when I actually did.
BTW, Goose, thanks for bringing that up. I agree with you.