Did you test versions of Dominion without Treasures? After all, Kingdom Treasures didn't come up until Prosperity, when the Treasure's strategic space had already been created (barring Harem, which is essentially a Silver with an end-game bonus), while Kingdom Victory cards existed from the start.
I did not test Dominion without treasures.
Treasures with abilities didn't appear until 2006. Which is to say, they were obvious and I had them immediately. Okay as a rough measure of how long it took me to try them: Harem is on page 3, Cache on page 5, out of the earliest 9-card pages of cards.
I felt that every set wanted a special victory card. I did not feel that every set needed a special treasure, and Prosperity had them as a theme. So I saved them for Prosperity, to be more exciting when they debuted. Then Philosopher's Stone snuck out ahead due to Alchemy getting pushed up. It was a concern for me at the time, was I ruining Prosperity too much by having that treasure in Alchemy. In the end, no, it did not ruin Prosperity.
Dominion has treasures because it was part of the premise. I kept them because they worked. I didn't try not having them, because I was happy having them. In general I do not focus on trying different things for a part of a game that's working great. I focus on the parts that aren't working, or on making more stuff.