[prod]

I think it depends on how well the car is put together from the factory. Of course you need to also keep up with stone chips and scratches.
These days rust usually starts where water and dust collect in tight areas that arent well protected from the start.
GM had an issue on many cars where bottom door seams rusted out from the inside because seams and welds were not properly sealed at the factory. Modern power windows allow a lot of water inside that will carry dirt and salt with it. Then it sits there and rots if not fully sealed. A simple spray of rust check will help with that if you do it from new. I usually do around the inside edges of panels, maybe on the underside if there are obvious places like frame holes and brackets holding steel lines.
But in most snowy areas, if its a "forever car" you simply dont drive it in winter. Get a $1000 winter beater.

[thundercrakcer]

I wouldn't do it, we do it for our army vehicles and I see how they do it. There will be oil or "rust proofing" everywhere. We have the green Silverado's you may have seen around, and they take out all the lights to spray inside that area, I was tempted do get my 5 gen done until I saw what they do after that it was a solid no. Last thing you will want is residue spraying around the outside of the car.
I did get the dealer package for the car protection, comes with under carriage protection which I'm glad I did.