I also think its unlikely that kernel people are not aware of the atomicity of rename. However, almost every post, including this one, avoid mentioning why rename is actually used, and rather use handwavy notions about application authors using rename because somehow we got hooked on it via ext3 behaviour, which is far from the truth.
I'm all for a reasonable API addition to implement O_PONIES, and would implement support for it in the stuff i work on (glib, gio, etc) the second it was availible. However, all existing applications already use rename() to do atomic renames() for reasons unrelated to system crashes, so why not just make all these applications work without additional changes? At little cost in performance.