Yeah let's just say that there is no such thing as a serious late game economy without ecumenopoli. I don't see how you could ever hope to compete with lousy tier 3 alloy buildings.
I've never had an ecumenopolis and I've dominated each game I've played. The minerals/alloys balance is a pain, but I've found that each AI player has tons of alloys available for direct trade. By mid/late game they seem to not care about food, energy, or minerals, but they place high value on the strategic resources (gases, crystals, motes). So if you ramp up production on those you can trade for additional alloys you need.