Aluminum, and composite body panels are nothing new. My 03 F150 has an aluminum hood. Several panels on my old SN95 Mustangs and GEN4 Fbody Camaro/Firebirds were either aluminum or composite. It's not a new thing. My wifes 17 F150, and my 18 F150's are holding up great. Aluminum can corrode, but comparing unprotected aluminum to unprotected steel in the outside elements, the aluminum with resist corrosion way better, unless you're talking about some quality stainless. I do prefer the bed being composite like how you can option on the 19+ Sierra's, and like comes standard on the 22+ Tundras. The composite bed is a good idea IMHO.
The big draw with the F-Series using aluminum was weight savings. It saved over 700 lbs on the crewcabs compared to an all steel body, and it reduced the F-Series weight a several hundred lbs, while allowing Ford to add weight to the chassis and other area's to beef them up without having to increase the trucks weight over the steel body trucks, or compromise on the GVWR. The new GEN13/GEN14 F150's only weigh 300-350 lbs less than the GEN12 trucks because Ford took some of the weight saving in the body, and gave some weight back to other area's in the trucks.