It's not mission critical, but it's important: in cold weather the windshield can fog up to the point of 0% visibility.

In hot weather it can become very uncomfortable or even impossible to drive without AC.

There’s a distinction between safety critical equipment and essential equipment. If the former fails, it could kill you. If the latter fails, you can’t drive anymore but you won’t die if it happens on the road. Brakes are in the former category, while things like HVAC and instruments are in the latter.

Safety equipment must not fail, but essential equipment can fail as much as your customers will tolerate.

You can steal a trick the motorcycle community discovered a long time ago, some water + soap on the windshield prevents condensation.

Our '83 Vanagon doesn't have AC, it can be uncomfortable but sure isn't the end of the world.