I have this theory that goes: over time Dominion strategy has shifted towards later, faster greening as the player base got more experienced, and engines more viable.
Earlier on, you'd have to cope with green decks much longer than we usually do now, and this explains why early Victory skippers/sifters are on the pricy and (now) useless side of the spectrum (Adventurer, Scout). It also explains how Scrying Pool ended up being much cheaper than Golem.
That was all for me guessing what goes on in Donald's and the Dominion playtesters' minds.
While some of what you say is true, that isn't really the explanation for Adventurer.
In the early days cards often did not get tried in very many forms. After an initial period when cards were way off, there was a period where I would make a card, guess at what it should cost and how best to phrase it and all that, and then if we were buying it and it didn't seem unbeatable, it would never change. While I liked the idea of a card being as cheap as it could be, I did not actually test cards at cheaper costs if we were buying them.
Adventurer is one of those cards. It cost $6. We bought it. Done. Scout, same deal.
Those two cards also had shorter lifespans than some. Scout was one of the 5 cards added to Intrigue to get it to 25 cards; Adventurer was from a batch of new cards after RGG took the game.
You get better at these things as you go along. Seaside is more polished than the main set and Intrigue; Prosperity is more polished than Seaside. Still if you ask why say Trade Route doesn't cost $2, the answer is, I didn't even consider it. It cost $3 and we bought it. I wanted to have no $2's in Prosperity, but I never thought, "I would make Trade Route cost $2 but I don't want any $2's."
Gradually the cost of a working card got more scrutiny, perhaps in part due to the weight of all of the published cards. In Adventures there's nothing that just got a free ride based on us buying it. Some of the fiddling around involved things like, how many $2's should there be, but e.g. Coin of the Realm cost $3 once, Dungeon cost $4. And it's not like you wouldn't buy them at those costs.