The S&P 500 Index looks vulnerable. What investors need to know.

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

The index decides the fate of more than $ 5,000 billion in investor-related assets. My only exposure is in my retirement accounts, spouses, academics, health care 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 fine print.

The…