The fact that a single cost reduction that's restricted to treasures is weaker than coin but stacked cost reduction become stronger than coin, restricted or not, is the part I like about the card. That was the idea.
Your last statement is kinda weird and off though. I don't think I can easily imagine a game in which Quarry Outpost is the only Action card being played. That would mean both players are only buying Labs and Treasures where the treasures are cheaper (or maybe one player buys only treasures). Surely a terminal upgrades that deck, if Quarry Outposts are out often the treasures might be more attractive often times, but 1 opposing Quarry Outpost and 4$ to spend or 0 opposing Quarry Outpost will come up and buying a terminal will be much better.