Some of this does make sense. Most windows users don't 'upgrade' their Windows version after all. They tend to buy a whole new computer. I don't mean enthusiasts, of course, but then half of those will pirate Windows anyway. When you're talking average home/work computer, however, few users will be bothered to buy an upgrade and even fewer are willing to pay the exorbitant upgrade prices Microsoft charges. They're more likely to lump it with what they've got until they buy a new computer.
There comes the point though, every time a user comes to buy a new computer, they will have the opportunity to consider buying a Mac. If they've had an unpleasant user experience so far then they could edge towards the Mac side even more. This has less to do with Windows 7 and more to do with the user experience your average user has had with Vista/XP over the years.
A new Windows release has the result of boosting PC sales - this is also likely in part due to PC manufacturers taking the opportunity to pump out extra advertising for users to buy new computers with the new OS. Hence more average computer owners will get it into their heads that it may be time to buy a new machine than usual. This can also lead to an increase in Mac sales as a by-product since some users will think "Actually, do I really want Windows again? Windows 7 is meant to be better but... I've really hated Vista so maybe when I buy a new computer I'll try something different."
Keep in mind that what I've written above is not typical of the average user on here who is far more likely to buy a Windows 7 upgrade and is very unlikely to buy a Mac ever. I'm not talking about gamers, I'm talking about the average user, the average college student, the average housewife, the average parent etc.