Getty Images / Spencer PlattGoldman Sachs and JPMorgan predict relatively muted S&P 500 returns over the next decade.RIA Advisors' Lance Roberts says the banks are right.High valuations and potential inflation could lead to lower stock-market returns, Roberts says.Much has been said about the gloomy outlook for the S&P 500 that some of Wall Street's largest investment banks conveyed in recent weeks.Goldman Sachs said in October that the S&P 500 would return 3% annually, on average, over the next 10 years, underperforming current 10-year Treasury yields.