30 years is such a long time. It's absolutely possible to build a device that lasts that long. But it is also entirely possible to mess up somewhere, and experience a failure way before then. In some cases, it could be repairable; in others, it would effectively total the device.

And you have no idea what this hypothetical future failure mode looks like. It could be as simple as a broken plastic bracket. It could be a dried out capacitor that isn't easily accessible. It could be a burned out motor winding. It could be a seized up bearing. Or it could be CMOS chip that had insufficient ESD protection.

I agree with you that DC motors are generally considered superior. But the devil is in the details. And you don't really know which model has had more attention to important details.

My NuTone ceiling fan (Casablanca type) has been operating since about 1990. It still operates but there is a slightly perceivable noise when each burst of electricity is expended at the lowest speed setting. It might be related to the bearings, or to loosening of the windings, so perhaps expecting more than 30 years is optimistic. However, for earlier designs such as in Rick's Cafe American, such fans may be designed to provide means for oiling the bearings, and if properly maintained might last significantly longer.