“Perhaps waxing every year before the first snowfall might add a few years.”
They rust from the backside of the sheetmetal out.
Also the undersides rust. Best you can do is take them to a carwash that cleans the underside, but that doesn’t get it all. If they put salt on the road and you drive on it, your vehicle is going to rust no matter what you do.
I have seen some very encouraging 10-year inspection videos on the aluminum-bodied Ford pickups, though. Seems like even the steel chassis rails are surviving, since it's easy to flush those clean in the Spring and re-apply your protectant of choice.
Aluminum may not be a solution for unibody construction, but the body-on-frame vehicles are offering a glimmer of hope.