Second what WW says. Taken to the logical extreme, it's like this:
Card
$5 - Action
At the end of your turn, if you have one of these in play, you win the game. If you have more than one, you lose.
One is so powerful it's a must-buy, so you have to buy one. Two isn't, so you don't. There are already cards like this. You rarely rarely rarely want more than one Contraband, Moneylender, Chapel, or Forge, so you just don't buy more. But the capability of those cards in singles is nevertheless appropriate for their price, which means none of them are must-buys.
If you do want to do a reverse City sort of thing, the way to do it would be to set it up so that even one copy degrades over the course of the game. The new Sage card is a good example: as you green, it stops finding your good cards. A reverse Trade Route mechanic would work, too -- that would do less and less the more tokens there are on the Trade Route mat. A true reverse City (the card degrades when supply piles empty) would be trickier, though, since a great many games end on Provinces or Colonies.