The market and the EPA demand vehicles with better fuel economy. All the low-hanging fruit was picked from that vine years ago; if you want to make significant improvements in fuel economy, you have to take a LOT of mass out of the vehicle. Manufacturers have known aluminum body panels are inevitable since at least the eighties, when they started to get serious about gas mileage. The steel industry has been able to hold off aluminum for thirty years with the development of high-strength steels, but you can only roll steel so thin. The transition to aluminum will be slow because manufacturers are tooled up to work with steel. But within the next twenty years I suspect aluminum will overtake steel.
The driver to go to aluminum is fuel economy. Corrosion resistance is a side benefit. In fact, some of the strongest alloys have poor corrosion resistance. But here in the Rust Belt, truck bodies usually rust out before the chassis and drive train wear out, so if an aluminum body can extend the life of a a vehicle by fifty percent, that makes it well worth the five percent added cost.
Although mass reduction is important, lowering the vehicle CG is becoming increasingly important. The market wants tall vehicles, but those same vehicles land manufacturers in court over product liability from rollover accidents. If you want to increase rollover resistance, you have to lower the CG, and there's not much mass to work with that's high enough to have a significant impact on CG. Once you go to an all-aluminum engine, the next logical step is to reduce sheet metal mass. I suspect for Ford, lowering the CG is nearly as important as reducing overall mass, given the company's history with rollover lawsuits.
As for repair, when high-strength steel came along Bubba had to learn new repair techniques. He'll either learn to repair aluminum, or he'll get out of the business.