If an insurance company is not allowed to factor in risk but instead is forced to cover everyone, regardless of physical condition, and charge them all the same rates, it ceases to function as an insurance company. Instead, it becomes a government-directed utility that just pays people’s medical bills. This is not really any different from government-run health care; it just uses private companies to administer the benefits. This is the point made by Charles Krauthammer:
By essentially abolishing medical underwriting (actuarially based risk assessment) and replacing it with government fiat, Obamacare turns the health insurance companies into utilities, their every significant move dictated by government regulators. The public option was a sideshow. As many on the right have long been arguing, and as the more astute on the left (such as The New Yorker's James Surowiecki) understand, Obamacare is government health care by proxy, single-payer through a facade of nominally “private” insurers.
Perhaps that’s what we want. If so, we might as well just adopt a nationalized one-payer system and cut out the middle-man.