This card would need to be priced at $5 or $6, so that it can't be spammed with Workshops. If you're going to buy 10 of them, then the other player has a decent shot of buying 5 duchies--for the win--while you're buying cards that don't help yourself at all. In the meantime, how are you going to regularly get to $5 if you are frequently drawing one or more of these cards dead? It's hard enough to get 5 Labs or Wharves--and those are cards that help you get to $5 to get more of them--but this card hinders you from getting to $5.
If you build an engine that can play one of these cards every turn, then yes, you can guarantee your win--just like you can guarantee a win with a Goons/Masquerade/King's Court pin or almost guarantee it with a Golden Deck (or its Platinum variant). But building such an engine takes a long time--in the meantime, your opponent can be buying reactions (Moat/Lighthouse), or VP chip cards, or Duchies to go for a three pile win, or some other exotic strategy (or he can just go for 3-4 of these, enough to keep you from playing one every turn.)
Remember that on the turns he can't buy VP, he'll be able to buy Gold or good engine parts instead--so when he does have a chance to buy VP, he'll be able to buy a lot of them.