I'm with the camp that my primary bike pre-dates all the modern frame savers by at least a decade or two. I do have rust on the outside, and I suppose also a layer of rust on the inside, but most of it is superficial, I think.
Here, we have no road salt, and it is not a marine environment, although we do get some winter moisture.
I probably won't start using the frame savers or internal treatments, although I might consider it if I was in a more salty environment.
I would consider something like WD-40 as great for a short-term treatment, but I would have to doubt its effectiveness for long-term treatment of years or decades.
My new "rain bike" is Titanium.