vans always have a much lower tow rating, the chassis and drivetrain are almost always different in some critical way

Apples to apples, those vans are more expensive and larger. At what point does it become ok to have an open bed? There are valid pros and cons in every choice being talked about, here.

You would trust your lungs to vehicle manufacturers getting every van air-tight with seals to last for many years?

Of course they would because they don’t need one for what they do. But I also imagine another few million around the world do. What you choose depends on what you need