Yesterday at Drupal Downunder, the question of product life cycle came up during the Q&A at the end of Dries' Keynote.
Traditionally Drupal supports the current version, and the previous version. When a new version comes out the oldest supported version is retired. This is great, except the unpredictable life cycle means that clients investing in Drupal sites cannot be certain of the amount of time before their site will have to be upgraded, and in some cases the support period for each major version number is not long enough (from a client perspective).
Take Drupal 6 for example. It's been 3 years between Drupal 6 and Drupal 7. Let's say the next 3 Major releases are 18 months apart each. While Drupal 6 will have enjoyed a 4.5 year period, Drupal 7 will only gets a 3 year run before the site owner will have to invest to stay up to date.
What if each Drupal release had a minium lifetime support guarantee? This gives the client confidence that the site they are investing in will be supported for at least a defined period of time.