Windows 8 may be a large UI change. But look at the history. The Windows UI has remained essentially the same since Windows 95. That's nearly 20 years.
Mac OS X was a large change from OS 9 but since then its desktop has remained pretty much the same. It has added a few features and changed a few minor things but I could be just as comfortable using OS 10.3 as 10.7. (10.0, .1 and .2 were a bit too clunky).