"[A] finding against the tax credits would mean that as premiums for coverage through the federal exchanges rise, premiums outside the exchanges would rise as well. This could make insurance 'unaffordable' not only for low-income individuals who would have qualified for the tax credits, but also for those who have relied on the non-group market for insurance -- the self-employed, individuals employed by small businesses that do not offer insurance coverage, early retirees, and those in-between jobs."