S&P 500 Index Is Looking Vulnerable. What Investors Should Know.

S&P 500 index funds will tumble by Christmas, one Wall Street strategist predicts. Not necessarily, says another—but they’ll lose money over the next decade. I can’t decide whether to panic or just sulk.

The index decides the fate of more than $5 trillion in linked investor assets. My only exposure is in my retirement, joint, college, healthcare, and, come to think of it, all other investment accounts. I don’t think my Chipotle Rewards account is affected, but I haven’t read the small print.

The…