But that once reasonable assumption on Wall Street is turning into a disaster.

Goldman Sachs is the latest firm to warn its clients that tumbling oil prices are bad news for stocks.

"We lower our S&P 500 [earnings per share] forecast by $3 to $106, $117, and $126 for 2015, 2016, and 2017, reflecting annual growth of -7%, +11% and +8%, respectively," Goldman Sachs' David Kostin said. "Energy is the leading driver of our reduced profit outlook."

Kostin follows Deutsche Bank's David Bianco and RBC Capital's Jonathan Golub in issuing this type of alert to clients.

"This seemed reasonable several weeks ago," Bianco said regarding his assumption that oil prices would average $55 a barrel in 2016. "But now it doesn't."