No, we can't. Yes, it needs to exist. If the enemy jungler is really doing nothing but farming for 20 minutes and you're not winning by a landslide, your jungler is doing something wrong. If your jungler is doing nothing but farming for 20 minutes, tell them to knock it off (politely).
As for junglers doing just fine without devourer... Shyvana was playable but suboptimal with Warrior or Cidnerhulk, Yi's never been doable with anything but Devourer although the old version did him somewhat okay. Nocturne's been bad for a long time and is only barely back, Kayle hasn't been a decent jungle pick in quite a long time. Jax wasn't seriously considered a jungler before, but now he's quite formidable. This item did a lot to expand the number of options us junglers have at our disposal.
(Also, it's a bit disingenuous to say "it's just like Feral Flare and wasn't that a mess". Is Devourer a mess or isn't it? It hardly matters whether feral flare was, since this is a completely different item.)