I couldn’t agree with you more on all fronts. I was with Toyota when they signed the multi-billion dollar frame recall on the tacomas. It was a weird time and Dana was made very aware that they were under investigation for their “galvanization” methods. That’s a whole campfire story in itself.Sounds similar to the first gen Tacoma frames. The c-channel area from the rear leaf springs back is usually where the rot starts; usually from the inside out. Although I don't know for sure, Toyota really seems to have been more diligent about rust-proofing on the 80 series, perhaps due to customer complaints on how fast the older trucks rusted, both on the frames and especially the sheet metal as well. Maybe they also used thicker steel to make them more corrosion-resistant.
Ironically the 100 series undercarriage appears to rust more aggressively than its predecessor, at least from what I've seen. I'm not sure on the 200 series.
I have seen so many 60’s around the Midwest and almost all of them have a good scale of rust at the minimum. I am not sure exactly how Toyota managed to increase the corrosion resistance of the 80 so well, but I’d put my money down in saying it’s the most rust resistant Toyota truck frame ever made. The 100 loves to rust right above the rear axle where water pools at the crossmember. The 200 seems to hold up much better as far as a rail rotting, but the damn KDSS valve assembly and all of the bolts that go into the frame love to snap off no matter how careful you are. The 80 is far superior to work on and how well it holds up.