When Air Bags first came out replacement was suggested at 10 to 15 years . Any air bag after 2000 is considered good for the life of the vehicle . This according to Mr. Google but you are welcome to web search for info that confirms or disputes that.
I agree that metal fatigue has probably not reduced the structure’s safety. Rust, however, may have done so.
Also that newer cars are a lot safer in a crash, by and large. This due to many factors, including better engineering of the structure, more use of higher strength steel where warranted, etc., etc.
If you had a reputable mechanic look at the car and he gave an OK you are good to go. Surface rust is normal
If you can get one…
When the ‘rocker panel’ unibody is dangerously rusted it seems to first become apparent with the door not fitting properly despite the door hinges being sound… But that’s just something that I have recognized in cars from the rust belt and they often continue to operate without sagging to the ground for several years once they reach the deep south.
An experienced, knowledgeable and HONEST body man’s opinion would be worthwhile if you can find one.
The last car I had that was dangerous because of rust was my 1957 Plymouth with unit body construction and very poor rust protection.
The front seat anchors fell through the floor. lowering the seat by several inches. My sister’s boy often asked his mother why Uncle Nick looked so short driving this car!
This is the same car that was buried as a time capsule in Oklahoma and dug up 50 years later as a complete rust heap!
Your 57 had a separate body and frame. Chrysler Corp. Introduced unibody construction in 1960.
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Here’s a good history of it