Re: Marie Antoinette. I think that just like in our times, there are people who are more motivated by following fashion than being modest and thus you can find innumerable images/paintings of these. I've read that Marie Antoinette was a flightly, vain young girl, but as she grew older she became more serious and devout. Maybe there are paintings of her older that reflect that change, likely shown by a more modest neckline.
As far as the resistance to longer skirts, I can only speak for myself and say that it's a matter of degree and wearing something so long is more extreme and therefore more noticeable out in public. Many of us don't like feeling exposed to public scrutiny and don't want to be thought of as "weird."
Plus, sometimes finding flattering skirts of that length and shoes to go with them is not easy. And then some of us, like myself have H who aren't exactly on the same page about what is appropriate and modest for a Catholic woman in the 21st century ... and you get this resistance.
As far as the matter of degree goes, it seems reasonable to have skirts below the knee, long enough so that your knee is covered when sitting ... and the skirt should be loose and flowing so as not to outline the figure.