AFAIK Windows 8 will also include a traditional-looking desktop environment for people who need to get actual work done. I simply can't imagine using Excel, Visual Studio, or Photoshop on a tablet. Also, I wouldn't be surprised if some of the more radical changes in Windows 8 got watered down in Windows 9. Remember, Microsoft caters to millions of corporate customers, and corporations need their employees to keep using familiar productivity applications because training costs a lot. Most corporations will skip Windows 8 and bombard Microsoft with productivity suggestions for Windows 9, which will be more mature. Besides, Microsoft already has a tendency to treat every other Windows release as a sort of beta (cough Vista cough). Odd-numbered versions are like LTS releases.
More likely, once the touchscreen establishes itself as a stable market segment, smart people will realize that 10" portable Facebook browsers need a different UI from dual-24" productivity machines. Then the "make everything look like an iPad" fad will pass, and desktop UIs will begin to diverge again. /optimism