To be a real American luxury brand you need real luxury cars. The MKZ, I'm very sorry to say, is not a real luxury car. It lacks an essential quality that real luxury sedans have. It's something called "sophistication." It's that feeling that you are floating serenely a few inches above the earth and yet firmly connected to it and in full control of the craft you are piloting. A real luxury car is like the hero who never loses his fedora even in a fistfight.
The MKZ, by contrast, feels rumpled and out of sorts. It looks nice, but it drives cheap.