Here is the article about 'Rust and Decay'
Stainless Steel isn't rust proof. I would say for being 55 years old, it is holding up pretty well. The issue with the arch is there is no way to clean/repair the outside of the super structure. Also, since the outside is actually what holds it together repairing or replacing pieces is near impossible.
Surface scaling and patina is a natural process, look at the statue of liberty, it isn't painted green, that is natural weathering.
I will love the arch no mater what color it is.
There is no excuse for the rust on the inside though.