ObamaCare will lead to a dramatic decline in employer-provided health insurance—with as many as 78 million Americans forced to find other sources of coverage.

This disturbing finding is based on my calculations from a survey by McKinsey & Company. The survey, published this week in the McKinsey Quarterly, found that up to 50% of employers say they will definitely or probably pursue alternatives to their current health-insurance plan in the years after the Patient Protection and Affordable Care Act takes...