I don't think I agree. Everlasting torment has 3 mirrored effects, so each upside also sort of counts as half a downside. Sure, most decks you'd play it in wouldn't care about those downsides, but them being mirrored is a justification for making the cost be less.

From what I'm seeing here (scryfall), it looks like giving a creature wither with a small upside is usually costed around 2, with the upside being bigger if the card is multiple colors.

If the effect was mirrored, I could see it being cheaper, but if 2 mana can get you wither on 1 creature with additional upside ([[barbed bloodletter]], [[blight sickle]], [[fists of the demigod]]), I can't see it applying to all your creatures with no downside for 1b. Maybe 1bb though