Investors' excitement should come from the index's long-term performance.On Oct. 29, the S&P 500 hit an all-time high, reaching just over 6,890 (indexes are measured in points and not dollars like stocks or ETFs). As of the opening bell on Dec. 8, the index sits at 6,870, flirting with its all-time high.On one hand, this has been great news for investors who've owned shares of an S&P 500 ETF along the way, and some have leaned into the momentum. On the other hand, some are growing concerned that the index's ...