I see a lot of vehicles with severely rotted frames like that,yet the body is absolutely perfect...and vice versa,I see a lot with perfect frames that have the body rotting off it too...it is weird how rust seems picky as far as what areas it will attack--I think the composition of the steel itself has a lot to do with it--if a frame or body part is made of previously rusted recycled steel,it rots away very fast and completely..practically nothing today is made of "virgin" steel...