I was only thinking of security updates, new features can often be added by third parties and that leads to a healthy market that will fill the gaps on older OS

I was only thinking of security updates, new features can often be added by third parties and that leads to a healthy market that will fill the gaps on older OS versions. HN completely ignores the class of computer users who only need Safari, Mail and some variation of Word, and then live in these apps for years without any major upgrade. In my experience, these people don't even feel entitled to any upgrade and they don't add any complexity to any testing matrix. All they need are baseline security updates, and existing services better don't stop without a plausible reason (see MobileMe). I'm not thinking of MegaCorp here, but rather my own and friends' close relatives. The machine that got excluded from 10.8 after only 4 years was not a rev-A MacBook Air in the hands of some early adopters but a Mac Mini, the most traditional Mac money can buy. That really leaves little hope that all my "slow adopter" relatives will ever leave the Windowsverse.