Obviously, if you’re not worried about playing Standard, Glimpse the Unthinkable is better, milling 10 cards for only 2 mana. But it's also expensive moneywise - $30 a copy! So if anything, Archive Trap won't be replaced. It would be Glimpse the Unthinkable for those that can't afford it. Alongside popular Mill cards like Mind Funeral and Mind Grind, having a couple of copies to replace Glimpse or less efficient mill cards may be worth it.
Yes, Archive Trap is better from a pure efficiency standpoint. But it wouldn't be replaced in the decks that want it anyhow. Startled awake was designed to be a limited bomb that players who love mill would gravitate too. It may actually make the entry cost of mill decks a bit lower, too.