Whether you care about how long s939 will be around depends on how you do your upgrading. I tend to always switch out my motherboard and processor simultaneously, making socket dilemmas mute.
Also, for me, upgrading about once a year is more fun than having the latest and greatest once every four years. If you are concerned about what the future will bring, stick with mid-range components that give you the most performance for the money. That way, you can change out your system to a new platform without feeling too guilty about it 🙂