What you are saying about the smaller 4-cylinder turbo cars makes 100% sense, and most people would think that is the way to go. However, sales of the new Falcon with the new EcoBoost engine have bombed in Australia, and I believe in NZ as well. It seems the car buying public still have an issue with a "small" engine in a big car. The same can be seen in NZ with BMWs like the 520i, good car, drives great - but is still seen as needing a bigger engine.
The economy of the car is usually off-set by the amount the companies back the bigger engines on the fleet deals - it used to be MUCH cheaper to lease an XR6 or XR6 Turbo than a billy-basic XT over a three year lease period. As for diesel, neither Ford AU or Holden have access to a half decent engine unfortunately - try driving a diesel Captiva or Territory.
So, yes the reliance on building a single "large" model car is a dangerous position to be in over the long-term. Personally I really hope they keep the plant alive, and invest some real money to bring it up to world-class standards. Once it's gone, it's gone forever!