Can your spells be boosted to affect enemies of all levels, or does it eventually cap, rendering your magic useless?

With Kindred Mage + Hypnotic Gaze/Aspect of Terror/Rage + Dual Casting, you can pretty much affect every humanoid.
With Dual Casting + Perks, Frenzy caps at level 79, Pacify at 83 and Rout at 88, which should be more than enough.
The lower level versions of the spells can still affect people around level 50-60 if you dual-cast them.

If that's still not enough, you can become a vampire or drink some Fortify Illusion potions.