I had a tech support guy tell me that Microsoft's official stance is that new OSes will be released every 18 months, but old ones will only be supported for 4 years - This is to force an upgrade every 4 years.
- Except it's been 4 years since XP came out
- Except Microsoft's own published timeline shows that their products (including operating systems) have 5 years of 'mainstream' support, and an additional 5 years of secondary support. Meaning during the first 5 years you can expect bugfixes and possibly enhancements / new features, and the next 5 years only important seucirty patches. After 10 years, you're on your own.