Windows 8 will be the most "successful" $$ OS for Microsoft .. and there are a couple of reasons for this:
1. It will come preinstalled on every laptop, AIO, tablet, OEM desktop (larger install base then all the previous windows could muster since it includes tablets now)
2. A larger install base means more licenses sold therefor means more $$
3. As a by product of such a large install base, ppl that want to downgrade to windows 7 on their desktop/laptops .. will pay for a W7 license so ... more cash to MS either way.
The bottom line is, on paper, it'll set a new record on copies sold in bulk .. but when you filter by platform .. well .. who cares right? Money in the bank!