Then why is stainless steel so resistant to drilling with hardened steel drill bits if it's softer?
Try to dent some 1/4" stainless sheet with a hammer. Now, try to dent some 1/4" mild steel sheet with a hammer. Guaranteed, the stainless is harder. It certainly resists rust for FAR longer than mild steel as well, and with nearly any grade of stainless alloy, it will easily outlast any of us, even if left outdoors for it's entire life.
Most stainless steel alloys don't rust easily at all unless you introduce an inferior metal in to it via welding.
It's also much harder to bend/shape/form, and it also work-hardens much faster than mild steel.
Sorry man, but I'm not buying it. I worked with stainless steel alloys in a production environment for decades, as well as both hot/cold rolled mild steel, and several aluminum alloys. Stainless steel is superior to mild steel in every way where longevity is concerned.