First thing I did with the '02 Wiinie I bought that had original owners from Minnesota was to apply by brush, and squirt bottle, Jasco Prep & Primer. It's phosphoric acid I believe. Whatever it is, it changes the rust back into metal...I checked on it before application, turns out it's been used for over a century as a rust preventative and stabilizer. Not as strong as the original metal after treatment but at least it slows the progression of rusting and prevents future rust. Costs $8/qt so not an expensive choice either. Using the squirt bottle, and applying it liberally, I still have part of that first quart I bought.
After 3 years, no further rusting. There were a couple places where the rust was bad enough that I hit it with a brass rotary wire wheel before treatment and brushed it on heavily but for the most part, the rust is stable now.
Much of the rust was on components that are thick and not likely to rust through in 100 years. As long as I stay out of the rust belt and avoid their ridiculous application of salt to the roads that is.
You are right that perhaps they should treat the undercarriage with a rust preventative but I've never heard of an inexpensive treatment and that makes all the difference to them.