Right now SS3's end-of-life is tied to the end-of-life of the PHP version that it requires - 5.6. This is December 2018. We're targetting an October 2017 release for SS4 stable, which only leaves a 15 months window for people to upgrade.
For comparison, we had 33 months (2 years, 9 months) between 3.0 being released and 2.4 being end-of-life'd.
15 months is too short a window and we would like a minimum of 2 years.