Based on the wiki information about the level range adjustments of illusion perks, if we work out the math for the Fury (novice) and Calm (apprentice) skills, it seems that a player could continue using these early spells indefinitely, provided that most enemies cap out at around level 50 from what i've heard.

Remember, dual casting (more than?) doubles the level range of illusion spells, AFTER all other perks are added.

So, Fury at base level works on enemies up to level 6, and up to level 14/16 with animage/kindred mage. Rage perk adds 12 to this range, for a total of 26/28, BEFORE dual casting. With dual casting perk, the spell theoretically is effective on all enemies up to and slightly beyond level 50, for a relatively cheap cost in magicka (roughly 2.8x the cost of casting a single Fury).

Calm is the same way; with all relevant perks, it would seem that it remains effective even towards high level enemies 50+, which raises questions:

1) has anyone progessed far enough to be able to verify this?
2) if the early spells can be perked to remain effective, why take the higher level and much more costly spells?
-just about the only thing i can think of is that higher level illusion spells are intended for "pure" mages, who will have enough magicka to use the higher level spells + dual casting to affect high level enemies, therefore not having to take all the other level adjustment perks (but then, they'd be missing out on Master of the Mind).
3) are some enemies always scaled to the player level, potentially also capable of reaching level 81?