An instant cast ability feels clunky in comparison to an ability with 2s cast?
Anyway, Doom fills two purposes: First one is making Demo's sustained AoE / cleave very powerful in comparison to their burst AoE. Another is increasing their sustained single target damage(Against bosses for instance). I would guess that they are intended to be relatively weaker in burst AoE and bursting single targets, or alternatively perhaps having no Doom would put them over the top in that regard. It's like an extreme version of a DoT, as a DoT on a burst target still ticks for instance slightly over half of its ticks and might even then be beneficial, whereas Doom will be entirely useless against a low health target. Actually, there's a third purpose: Casting something while having to move.
The accurate effect remains to be seen until they are done with the first tuning pass, but the general idea is that the purpose of Doom's existence is making Demo's sustained AoE and cleave a lot stronger, making Demo's sustained single target damage slightly stronger, making Demo's burst AoE significantly weaker and making Demo's burst single target damage slightly weaker.
What would you propose to replace it that would fill these purposes? Just removing the spell and buffing other damage to compensate would obviously ruin this balance.