I just realized that this is one of the very few things in Dominion that makes you want to get more than one Potion.
Yikes. I don't know, sometimes you need to concede the 5 VP if it gets too ridiculous; you'll just be adding more stop cards to your deck. I mean, who wants to pick up 6 Loans? 6 Contrabands? (though, I suppose even if you play 2 or 3 Contrabands, there will be enough treasures to buy still that your opponent can't ban all of them). Hah, this Landmark is ridiculous.
Well, better to add 5VP by buying a stop card for $4 than adding 3VP by buying a stop card for $5 (aka Duchy).
I actually think that there will be a couple of games where you run out "useless" Kingdom treasure piles, especially if there's no fast way to end the game.
Yes, but a Duchy is a guaranteed 3 VP for $5. You very well could buy 4 Potions and they will count for nothing, especially if the Potion card itself is a dud.
I don't think it really works that way. Consider this: You and your opponent both have the 0 VP, and 0 Potions.
Scenario 1:
You buy a Potion, get a 5 VP lead.
Your opponent buys a Potion, also getting 5 VP. Now you have the same amount of VP again.
You buy a Potion. This gives you no VP, but makes your opponent lose 5. You end up with a 5 VP lead.
Your opponent buys 2 Potions. You lose 5 VP, and your opponent gets 5. They now have a 5VP lead.
Result: You have 0 VP, your opponent has 5.
Scenario 2: (the same with Duchies)
You buy a Duchy, get a 3 VP lead.
Your opponent buys a Duchy, also getting 3 VP. Now you have the same amount of VP again.
You buy a Duchy. This gives you 3 VP. You end up with a 3 VP lead.
Your opponent buys 2 Duchies. They get 6 VP. They now have a 3 VP lead.
Result: You have 6 VP, your opponent has 9.
In scenario 2, you end up with more VP, but it doesn't matter because all that matters is the VP swing from each buy. And each Potion provides a 5 VP swing, while the Duchy only gives 3 VP.
Disclaimer: In multiplayer, things get more complicated.