The Affordable Care Act was built on what its designers thought of as a three legged stool: one leg was the requirement that health insurance companies cover all individuals no matter their pre-existing conditions, one leg was an incentive for those individuals to purchase health insurance by providing them subsidies, and the other leg was a punishment if those individuals refused to buy insurance (individuals who didn't buy health insurance would be fined hundreds of dollars per year). The authors of the bill were insistent that all three "legs" had to be in place or else the market for health insurance under the ACA would collapse. As part of the Tax Cuts and Jobs Act passed in December 2017, Republicans removed the punishment for not purchasing health insurance. The penalty (which used to be hundreds of dollars) was reduced to zero dollars and yet, the ACA health insurance market didn't collapse. What might explain this?