Yup, the question of when to replace in some areas is not so much about use, or mileage, but rather when it rots apart and isn't safe anymore, or has so many things going wrong due to rust and corrosion that you just can't keep up.
When I lived in MI, I had a '98 Sonoma Highrider that I sold in '04 because it was starting to rust pretty good at the tailgate and bottom of the doors. It was very dependable with 120k miles on it, but I was worried that in another winter of rusting, its value would have been a lot less. I replaced it with a '96 K2500 6.2L/5 speed with 190k that I bought for cash. It was a bit beat, but a good truck, and that's all I wanted. By the time I got rid of it four years later, it was a rusty mess, with the rockers pretty much gone, though it was still a dependable truck with 290k miles on it.
I replaced the '96 with an '06 Duramax that my uncle had used as his company vehicle. (Oil field business.) It had had a plow and salt spreader at one time, and was a rusty mess underneath, but it cleaned up well. I owned that truck for not quite a year before I sold it to some truck place in WI.
I escaped the rust problem when I left the rust belt... I bought the rust free '06 GMC Duramax that I have now from ND in Sept 2009. I lived in MI though that winter, but I only drove it when the roads were clear, and I washed it REALLY good underneath if I did drive it. Spring, 2010, I moved to CO, then to NM in 2014. That truck now has 240k miles (and ~8000 hours!) on it, and no more rust than the day I bought it. I finished payments in 2012. Since then I've put an alternator on it, and a set of tie rod ends and steering/pitman arms. Oh, and I did the brakes at 220k miles because I had them on a shelf and was tired of moving them... Of course it gets tires when it needs them, and lots of fuel, but it gets 20mpg, fits me, and I like it. I don't care for styling on anything newer, so this is my truck until I wreck it, or die... ??
