You're making an assumption that the second the warranty is up, the car falls completely apart??
I'm just saying a 5 year warranty rust warranty does not inspire confidence in long term rust performance. And, there are usually limitations on that like "outer body rust through".
I see quite a bit of outer body rust trough on vehicles less that 10 years old here in Michigan. Usually they are far worse underneath by then.
Rusty vehicles are difficult/expensive to maintain, and eventually completely unsafe to drive. I just bought a good running E150 Chateau for $450 because the frame rusted so badly the forces from steering box literally broke the front of the frame completely off. I like to drive beaters in the winter to save my nice vehicles (not the $450 E150 - that's for parts only), but I know I'm increasing my risk of injury from accidents substantially. No one tests the crash protection of rusted vehicles. I bet it would be scary. And, what about the risk of being hit by others with suspensions about to fail on the next bump. If Michigan had inspection laws, hundreds of thousands of vehicles on the road today would be scrapped.
Even a ten year rust life is unacceptable to me. If it was reduced to scrap I'd still be at $1/mile depreciation. Rust is a serious concern for me any many others who aren't using their vehicle for expediter/delivery service. That said, I've looked underneath a couple daily driven 2014s and they don't appear to be rusting any faster than the average vehicle. I't only been 2 winters though. And lots of 2014s were sold in 2015, so only one winter on them.