Or maybe you're being an obtuse jackass because rain of fire is most certainly a mechanical downside of the spec compared to a fucking channel that follows mobs as they're moved around and doesn't cost them their spender resource in competition with devouring plague.
The damage of rain of fire has been fine and viable even in BfA at higher target counts. Destro is still garbage in M+ because in real scenarios the mobs don't end up eating the full channel of ROF. It's that simple.
The likelihood of downtuning all shadow aoe spells and talents is far less than tuning down the outlier covenant ability in their flagship expansion feature. That too is the most obvious thing in the world, though apparently not to you.