If you're on 3 mexes, you've lost. If you're trapped on 3 mexes the enemy will have way more mexes than you and thus can build more metal makers much faster than you ever could- and since this kind of growth is exponential, they'll be way ahead of you on the curve. Metal makers won't save you in this situation.
We want players to be able to invest in their economy, but eventually they're going to have to move out and fight for new territory. This is what overdrive does: It keeps territory important while still allowing economic investment. It's a war game, so you're going to have to fight for resources.