Krisanapong Detraphiphat/Getty Images; Jenny Change-Rodriguez/BI IllustrationGoldman Sachs forecasts muted S&P 500 gains, with a 3% annual return over 10 years.Mega-cap tech stocks and AI enthusiasm have led to high index concentration.Diversification and equal-weighted S&P 500 exposure may offer better long-term returns.The S&P 500's bullish run over the past year, with an over 38% return, may come at the expense of its future performance, according to Goldman Sachs.An October 18 note, led by David Kostin, forecasts muted gains for the foreseeable future.