I agree with you frame should not rot out in 10 years. But from experience I can tell you that a replacement frame is not necessary talk to local racers, etc. and find the best damn welder in your area and most likely he will weld 3/8 to 1/4in patch panels to it. Now we all know that this only covers the rusted metal. However, a good welding shop will take the time to knock of the heavies rust, treat it with a rust converter, then weld in patches, and finally spray metal with a sealer/under coating. My 05 F150 FX4 had to have front passenger frame done, as well as both inside edges to rear frame just in front and in back of the rear leaf spring perch.
In addition, from the photos it looks as if your rust issues where the same as my Ford and all propagated along the factor weld seam. If this is the case and the metal is not compromised a full 360 in that area, then the frames integrity is still intact, but it's torsional rigidity is compromised until patches are installed.
I notice there is a lot of expectation that they will "cut and replace" in most cases like these that is not required and in most case not practical, and in some problematic. If you start cutting on the frame to remove all rust then you compromise the frame even further. In a frame off resto, that's fine as you have the body and weight off, in addition you can take measurements from every angle to assure its going back square. In this type of repair the idea is to slow and if possible stop the rust progression. Then to weld over a plate of steel that reaches far enough to be attached to good solid metal.
On my Ford, they had to do inner drivers frame beside gas tank. Taking the tank down just adds to problem and cost, so the weld a thicker plate up in beside the tank to span the problem area.
The steel they weld on will most likely out last the rest of the frame......