#1 A Windows upgrade is $99, typically, and includes restrictions about installation.
#2 A full version of OS X is $129 (not much more than the windows upgrade), is developed and upgraded substantially every 1 to 1.5 years (not every 6-7 years), and gets more than just bug fixes between versions. Plus there is no need for antivirus software. So, TCO:
7 years of windows:
$299 initial purchase
$210 7 yrs of antivirus
$099 upgrade to next version
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$608 TCO (plus pro system cleanups)
7 years of MacOS
$129 initial purchase
$516 4 substantial upgrades
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$645 for 7 years of "just works"
Pretty comparable, huh?
Upgrading, Why It Hurts To Be A Mac User