This is stemming mostly from the discussion around Ashiok, Dream Render from the latest episode of So Many Insane Plays. In the episode, it is speculated that Ashiok's mill with exile is powerful enough and plentiful enough that it could win games on it's own, ostensibly by removing wincons from your opponents deck and giving you inevitability in a drawn out game. Obviously Ashiok has a TON of other utility outside of milling cards which pushes him into a new category, but it made me question how we as a community treat mill on a whole.
Conventional wisdom is that strictly mill cards in the traditional sense are unplayable unless they win you the game on the spot, something like a painterstone combo. The logic is that because mill is off the top, anything that does not kill your opponent on the spot is just milling random cards, and that the mill is just as likely to mill your opponents good cards as it is cards they did not want to draw. Compound that with all the graveyard strategies in the game and mill is typically just garbage.
So it got me thinking while you cannot control what gets milled, your opponent put those cards in their list for a reason. There are games that players can look back at (Typically limited but it can happen in any format) where a card that milled cards from your opponents list won you the game because of what it milled, so while it is random it can be powerful.
I think the concept of milling directly into exile, should that become the new norm, possibly shifts our paradigm. By removing the possibility of giving your opponent graveyard fuel, you take away most of the risk inherent in milling someone. Playing mill cards that exile for the sake of potentially removing vital pieces from an opponents combo or synergies and providing inevitability could become worth playing. If they do you then just need to weigh if taking a chance to mill a key card and attacking a library is worth the trade off for playing cards that do not affect the board or stack. Is it worth spending mana and a card to potentially remove part of keyvault some of the time?
So here is my question. How many cards would a card have to mill directly to exile for it to be worth playing just for that ability (and not all the other things that Ashiok brings.) Let say you have this card:
Super Tomb Scour - U
Your opponent removes the top X cards from the top of their deck and puts them into exile.
What would X have to be for this card to be playable?