Health reform is the signature achievement of the Obama presidency. It was the biggest expansion of the social safety net since Medicare was established in the 1960s. It more or less achieves a goal — access to health insurance for all Americans — that progressives have been trying to reach for three generations. And it is already producing dramatic results, with the percentage of uninsured Americans falling to record lows.

Obamacare is, however, what engineers would call a kludge: a somewhat awkward, clumsy device with lots of moving parts. This makes it more expensive than it should be, and will probably always cause a significant number of people to fall through the cracks.

The question for progressives — a question that is now central to the Democratic primary — is whether these failings mean that they should re-litigate their own biggest political success in almost half a century, and try for something better.

My answer, as you might guess, is that they shouldn’t, that they should seek incremental change on health care (Bring back the public option!) and focus their main efforts on other issues — that is, that Bernie Sanders is wrong about this and Hillary Clinton is right. But the main point is that we should think clearly about why health reform looks the way it does.