The share of US companies that do not provide health insurance spiked this year, to nearly 55 percent. After next year, many New York firms may have to join them — or risk downsizing. Thanks to a 2012 state law, many mid-sized firms — those with between 51 and 100 insured employees — may have to drop their health plans, beginning in 2019. Wittingly or not, the state is pushing employers to purchase much more expensive and often less comprehensive insurance. Absent changes to