Light does show through the button. It needs to show through the button. When one aim the rifle using them sights what one 'sees' is the light coming through the fiber optic tube of the front sight, and through the button(s). This light has the diameter of the tube that is the front sight. The button isn't seen. The button is just some bigger than the tube itself. The tube of light is what is seen. Done correctly, the button will still be smaller than the outside of the steel parts that hold the fiber optic tube in place, but larger than the id of the steel component. The eye doesn't see the button, it sees the 'tube' of light transmitted by the fiber optic.

The super glue doesn't necessarily penetrate the fiber optic; it coats it. Coating the fiber optic tube makes it not so transparent on the ends, and dulls what one 'see' when sighting. It also hinders light from entering the sides of the tube.

The super glue bond will eventually fail. The button, if done correctly won't fail, unless it is banged/busted. The super glue bond will eventually fail.