One of the considerations in establishing the length of a warrenty is choosing the strength of the materials that will be used. Companies would obviously error on the side of strength to avoid the problems of a frame breaking. An example is the Chicago made Schwinn Paramounts (the ones before 1978 with chromed Nervex pro lugs). I’ve repaired/repainted them and the Reynolds markings on the tubes were 19/22. That is wire gauge and when translated is approximately 1.0/.7/1.0mm wall thickness. In other words the heaviest standard tubing Reynolds made at the time. That is perfect for a loaded touring bike or when a rider is 250 pounds or more but they ride like trucks compared to the lighter tubing I prefer to use when I make a frame for myself.

I vastly prefer the ride of frame that has a wall thickness of .7/.4/.7mm in the main tubes. I prefer the quality of that ride more than the titanium frames I’ve made. That is why I quite making ti frames. I recognize other opinions may differ and have preferences that vary from mine. This is why we don’t all choose the same color. I’m 5’ 8” (at least I was) and on the light side so my tubing preference for what works best for me would not work best for someone bigger. But that is the thing about making a custom steel frame. I can choose the diameter, wall thickness, tubing hardness and even where I place the butts to tune the frame to the ride quality i'm trying to achieve.

The problem with a frame made that light is that it is not as durable. It is much easier for bad things to happen when not “just riding along”. And steel rusts and if not properly addressed on a routine basis, can rust through in places like the underside of the chain stays. So establishing a warranty based on material choice can be tricky.

The OP’s wife is shorter and weighs a lot less than most. That is why I was suggesting a custom design might work best for her. What satisfies the accounts and lawyers might not her.