"well if corsets really weren't torture devices, why did they immediately disappear when bras were in invented?"
They didn't. They moved exclusively to the lower torso, got elastic, and became early Spanx but more socially prevalent.
The the 1920s just has really good PR as far as fashion goes and everyone keeps passing down the lie that it was some grand era of women's sartorial liberation because the Zeitgeist of every era is that what they have is better than anything that came before, and we are very willing to believe that when we read people saying it in 1920s magazines because it conforms to our cultural notion that "clothing that look more like modern clothing better."
People had said the same thing about fashion trends for at least a century at that point, but everyone ignores the earlier examples because they don't involve a dramatic shift from "clothing style that looks unimaginably foreign to modern eyes" to "clothing style that kind of makes sense to us."
hope this helps
