Blows the subscription model idea out of the water.
tl;dr: "supported lifetime" seems to be the easy cop out here, especially since it is a term that doesn't apply to desktops in the same ways that it does in mobile, and trying to force desktops to adhere in that manner isn't a good thing...
True, but it raises other questions instead. "supported lifetime of the device" is a highly suspect phrase here. We see Apple giving phones and tablets approximately three years of "supported lifetime". Apple can do that for a few reasons, but amongst the reasons why the customer base generally tolerates it is because Apple releases new devices mostly-annually, so getting three years of both hardware and software improvements is usually a worthwhile investment for consumers - the "supported lifetime" is acceptable because of trade-up.
Desktops are a completely different animal. I suspect that a significant minority (if not a majority) of iPad users have a desktop or laptop in active use that is older than their iPad, and I'd similarly suspect that the majority of them would prefer to upgrade their iPad this year, rather than their desktop, given the choice of only being able to spring for one or the other. What we've done with desktops is had two functional tiers of existence: in-warranty (where the OEM generally updates and fixes things), and out-of-warranty (where 'the computer guy' handles this stuff). Mobile devices are more disposable, in no small part because repairing them and upgrading them isn't always desirable - either expensive, impractical, or both. This is the area where traditional desktop/laptop computing has always had an edge.
Moreover, it is highly irregular for OEMs to provide OS upgrades to their computers. I had an HP laptop that I bought with XP after Vista's announcement (but not its release), so I registered the machine and HP sent me a Vista disk and key. That was highly irregular, and had to do with my purchase date, not my warranty length. I bought my Origin laptop with a three year, soup-to-nuts warranty in early 2011. That ran out last year, long after Windows 8's release. No Windows 8 disc in sight, and while admittedly I didn't ask, I sincerely doubt Origin would have sent it to me, despite the laptop being within the "supported lifetime of the device" by most practical definitions. The machine does, however, get regular security patches for the version of Windows that it /does/ run. Mobile devices' concept of "updates" involve both "security patches" and "OS version upgrades", whereas desktops do not.
So how does all this tie into Windows being a subscription or not? Well, both "OS updates" and "supported lifetime" are more clearly defined terms in mobile devices, and bringing that paradigm to the desktop yields yet another issue: the "broom problem". If you're a Doctor Who fan, there was an episode this season where The Doctor was saying that if you take a broom and replace the head when it wears out, and then you replace the handle because it breaks, and then replace the head again because it wears out, it's not the same broom anymore. With desktops, we have the same issue - the hard disk dies, we replace the disk. We upgrade a 4GB RAM module with an 8GB module, we add a video card to play games, and the PSU to power it, then we want to SLI, so we replace the motherboard, which mandates a different processor, and change the DVD burner to a Blu-Ray burner...If all these upgrades and replacements happen over three years, we end up with a completely different computer than the one we started with - at what point does the Windows license no longer apply? Microsoft has arbitrarily pointed to "the motherboard", which in fairness is about the closest thing one can get to a reasonable component definition (it's got the most hardware-as-detected-by-Windows of any single physical component), but even those die or have some sort of issue or whatever. What if the CPU is supported, but the motherboard is not? I sincerely doubt Gigabyte is supporting this board anymore, but it is, in theory, compatible with a CPU I purchase today from Newegg. Conversely, if I put an old IDE DVD-ROM into a computer that's otherwise fully supported, does it no longer count as a "supported lifetime of the device" because one component is no longer serviced?
I don't like it no matter how you slice it. Either Microsoft gets super-duper strict with component swaps in order to uphold the definition of "supported lifetime", or Microsoft lets basically-anything qualify in the hopes that revenue is made elsewhere (tracking, mobile sales, and "apps"). The third option would be that MS goes "subscription", which then raises the obvious question of "exactly how crippled will the computer get if the subscription isn't paid", a line so difficult for Microsoft to toe that the closest present analog - a copy of Windows that's failed activation - relies almost solely on nag screens and a lack of wallpaper.
Finally, it's not entirely inconceivable that Microsoft isn't trying a new twist on their old standby - Embrace (new technology, 'services first' focus), Extend (market share by making it possible for every user to get the new version that is largely integrated with the new services), and Extinguish (make "Free Windows" progressively less useful with the shiny new "subscription" option a click away).
It will be interesting...and I'm most certainly hanging onto my plastic disc editions, just in case.