Let’s take your example and assume that your opponent only draws one card a turn, and that the game lasts n more turns. n<=20 because we’re assuming the traumatize wasn’t going to mill you out. That means that the opponent was never going to draw their wincon if you didn’t traumatize them and it was in the bottom half. So 50% chance to get rid of their wincon, but 50% chance to give them access to a wincon they didn’t have otherwise. However, you’re probably right in practice. If you take into account draw spells you can go through your whole deck and still have more draws available to you and not get milled out, which means the mill card removed access to cards in your deck without directly killing you.
Note that in hearthstone it’s different because a) you have a smaller deck, so you’re more likely to go through the whole thing and b) you don’t lose when you run out of cards in deck, so removing cards from an opponent’s library is relevant in more situations where decking is not.