The investment bank's analysts simply recognize how much has changed in just the past few years, and what that means for the near future.The stock market's going to be dishing out subpar gains for the foreseeable future.That's the take from Goldman Sachs' (GS 1.56%) recent outlook, anyway. The investment banks' analysts expect the S&P 500 (^GSPC 1.56%) to log an average annual gain of only 6.5% over the coming 10 years, down from its long-term annualized pace of around 10%. But why? For some of the reasons ...