And they could be up to date for many, many, many years if Linux was installed on those, instead.
Well, if by "many, many, many" you mean "about five". Red Hat provides longer support terms, but not for free.
And then after those years shift from a "update" path to an "upgrade" path and have many years more.
True, sort of. Assuming the upgrade works, which it often does... but not always.
I'm no Windows fanboy, in fact the last version of Windows I used was Windows 2000. I switched to Linux completely by mid-2001 and I've never looked back. But it's really not as rosy as you paint it. The commercial OSes (Windows and OS X) actually do a much better job of delivering long-term support and (in the case of OS X) upgrades. I don't personally care about that for my machines, because I have no problem managing them, and even debugging the occasional update or upgrade failure. But I'm actually glad most of my relatives use Windows, because I would not enjoy having to help them do the same.