does anyone here actually think the fact that we play BM from time to time is a problem, or why are we discussing this?
The OP does. But yeah, I'm with you.
...and so does every fifth newbie who refuses to play Dominion with me.
There comes a point of expertise, after which we tend to forget that it's possible for any human being to not have that expertise. This thread has been a classic example. We actually saw someone speak as if he expected me to correctly predict the output of a complex system-- as if it were obvious in advance.
Perception matters-- even incorrect perceptions. Not just what you find out after your three hundredth game.
Funnily, none of the people I introduced the game to has ever complained about, or even played, pure Big Money, as far as I remember. It could be because I usually try to make "balanced" semi-random kingdoms IRL, so that BM is almost never competitive; or could "overpowered BM" be a group-think thing?
I agree that a casual player can't be expected to completely predict the output like LF did. But IMO it's still quite evident that your variant was very drastic: A Treasure bought in the early game is usually played at least 5-6 times, so each such Treasure would have a "negative Province" attached to it. Every Province bought using three Silver/Gold would effectively only be a Duchy, even in engines when there are no kingdom cards that give $.
Contrary to what LF said, giving a 1VP on-buy penalty for treasures might work IMO, at least if you allow the first 4-6 treasure buys to be free. It would hurt Silver and Gold less than an Embargo token on the pile does, which can easily happen in regular games.
I realize that "my" Hiespielchen proposal has a similar problem as your suggestion (on a smaller scale): It makes engines more competitive against BM on boards where both are possible, but it also drags out games where BM is the only viable strategy.
In principle, the best way to weaken BM would be to strengthen Action cards. Apart from reducing some cards' prices, as I suggested above, one might also give small amounts of VPs for gaining Action cards, e.g. by giving each player a "virtual Vineyard" (i.e. 1 extra VP for every 3 Actions) at the end. Since Vineyard doesn't break the game at all, this should make for a reasonable game, if skewed towards Action-heavy strategies.