I was gonna say something similar, I saw some pre-stressed concrete for part of a bridge used stainless cables for tensioning. Although it's not cheap as plain steel cable and iron rebar.
Although, stainless is not rust free, it's called stain-less, not stain-free. So it will degrade eventually especially in harsh environments.
It really depends on which type of stainless steel that is used. Some, like 400 series, will in fact rust.
Others, like Hastalloy and Ferrallium, have very good resistance to corrosion. A company I worked for used these metals for burial containers that were certified for ~300 years.
Although, stainless is not rust free, it's called stain-less, not stain-free. So it will degrade eventually especially in harsh environments.