Sigh. I stand corrected:
https://en.wikipedia.org/wiki/Wetting_current
However, on a slip ring device, the contact spring pressure is
sufficient to break through any oxidation. Therefore, there's no need
to rely on DC current to break through it.

That in fact is correct -- hence better quality dry circuit switches are implemented with a wiping action set of contacts AND a low-reactivity contact material such as gold.