Assume Windows 8 turns out to be a massive success. Suddenly everyone expects the same apps to run on their tablets and their desktops. Everyone expects to have proper multi-user support on their tablets, full productivity suites on ARM, and the option of buying an ARM laptop with 10+ hours of battery life. They expect these things because Windows 8 proved them possible.
Now what does Apple do? They've bet on single-user tablets, Intel laptops (with better-than-average but still not great) battery life, separate tablet and desktop ecosystems, and tablets that have only basic productivity apps. How do they bridge this gap and pay this debt?
I think Windows 8 is a very nice platform, and though it has some quirks, I think if people try it they will love it. I'm a Microsoft employee, though, so I'm probably biased.
They didn't replace OS X with iOS, they each exist as expressions of a different type of computing, for different purposes.
Microsoft are attempting to provide one system as a single expression of both types of paradigm, and this is why the jury is out on Windows 8.