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.
I dont think it is that different as to not be relevant. Hearthstone games on average last around the same number of turns as magic, sometimes a bit more. You don't outright lose in that game when you run out of deck, you do however start progressively start losing more and more life every turn because of it and eventually it will kill you. There are mill strategies for just that reason.
We regard a card like sadistic sacrament fringe playable because it is targeted mill, we know what it will hit, but there is not a gaurentee that the cards we remove would ever be drawn in the first place. Where as people dont play blind mill for this very reason, they do play sacrament.
I guess my question then becomes how much value does this type of mill add when it is incidental/value added to the card. If Ashiok did not mill but still did everything else, how would we regard him? I think its clear that the mill into exile adds something we ascribe some value to, but how much?