They've been doing that for years and have it down pretty well (though far from perfect). I upgraded my Ubuntu desktop with every new release since Feisty until earlier this year when I switched to LMDE, which is a rolling release and much less of a hassle. Each new update brought along a few problems (especially with ALSA/PulseAudio and WiFi) but they were all trivial to fix. Upgrading Windows is such a PITA that it's better to just format your hard drive and do a fresh install.
All in all, it's probably going to be a lot more trouble to upgrade Windows every year than to upgrade Ubuntu twice every year. I could also say that nothing's to stop you from sticking with the LTS releases and only upgrading once every eighteen months, but considering that almost every PC at my office still runs Windows XP, I have to concede that barring some wird policy shift in the future regarding backwards-compatibility and updates, Microsoft has the advantage in that area.