Will we always see a difference in brightness along an optic?

Not essentially so. The human imagine system appreciates illuminance grades into a logarithmic fashion and when the transition was smooth would be very complicated to actually notice the difference. When we observe an optic of, as 30 meters, from one end to the other along the optic this would not matter wherein end is situated the illuminator: we would notice the optic homogeneously lit, while we know that this is impossible. If we were to look at similar optic sideways, from several distances, then we would see the difference in luminance, since we could compare both ends.

The judicious utilize of illuminators, restricting the length of the fibers to which recommended by manufacturers, daisy chaining the optics, the angle of vision and the control of the contrast, are the tools required for a successful installation.