The first replacement name was "Windows 8 UI", which makes no sense for two reasons: 1) the UI was born in Windows Phone 7, not Windows 8, and 2) the term becomes extra useless when Windows 9 arrives. Unfortunately, the new name that Microsoft is using makes even less sense, because the company is using the term "Modern UI Style". Did no one at the company realize that it's terrible to use a term that changes its meaning faster than a rumor can spread about an Apple product?
The word "modern" means something different in almost every context, and in every successive week. What is "modern" today can be old hat tomorrow. Luckily, this likely isn't the official replacement term, but we're not really sure why Microsoft can't just stick with "Metro". We think that's a great term, because it hits the two major naming conventions: 1) it's timeless, and 2) it's abstract enough to not really mean anything.
That's exactly what you want when naming a product, just like Microsoft had with the "Aero interface", Google with the "Holo theme", or Apple with its "Aqua UI". All names that can't have vague connotations, but no connections to anything substantial, and certainly no overt references to time. Microsoft should be announcing the proper replacement term any day now, since it was supposed to have been done last week.