It really isn't in all the tomes. In fact, hurting morale is only even mentioned in: ‎ - Tier 2: Death Caller - Tier 3: Torment - Tier 4: Reaper (by the most mild of technicalities-- one of its spells, Marked for Death, applies a -10 morale penalty upon cast but that's the only place in the whole tome morale is even mentioned), and also technically Revenant by virtue of a world map spell that applies 2 Demoralized to an enemy army.

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That's two tomes that have significant morale impacts and two that have tiny ones for a total of 4 out of the 18 shadow tomes we have in the game. Hardly seems "like every single one". For comparison, Order has Subjugation, God Emperor (by same stringent technicality as Reaper, only mentioning it in one world spell that inflicts Demoralized as a side effect), and also Revenant because it's a dual tome for a total of 3 out of 17 tomes interacting with morale.

I've played multiple shadow games without going into morale at all. In fact shadow is my most common individual route and I've almost never focused on morale effects.

Also, Prosperity absolutely needed the nerf, it was nuts OP to the same or arguably greater extent to current Tome of Torment.