Wow I'm shocked to read these problems. I had a 1974 Datsun Pickup that rusted away before my eyes. Sold it in '82 and it was in the junkyard about two years later - fine mechanically too! My '87 Dodge pickup has some added ventilation in places, but is still roadworthy - it has never spent a night indoors.
I doubt our tractors are going to last 40 years for the simple reason that there are too many of them - too many makes, too many models, too many different parts. It's not like when there were 300,000+ 8n's out there and they were pretty simple machines. Now every manufacturer makes a dozen different models, and changes them every couple years. 30 years from now, you'll have to have most parts made, or make 'em yourself. It's a shame.
The rusting, though, that's almost criminal. No excuse, and I don't buy the "recycled steel" stuff. Steel has been recycled forever, and that's not the culprit here. My Dodge pickup might have some Model A in if for all we know. I know it's cosmetic, and my tractor stays outside, but the gray plastic in the fenders has faded a lot. Looks bad, and there's not much can be done. I put some of that no-wax floor stuff on it and it seems to be holding out better, but there is no reason not to have decent UV protection on these materials. Seems everyone wants you to buy new every five years, and for $40,000, if they can get it out of you, too!