Unfortunately because of how ultrasonic transducers work you need an extremely rigid connection between it and the blade - otherwise it's like having a spring between the two and it completely negates the benefits of being ultrasonic. The easiest way to get that connection is just to weld the blade directly to the transducer. In a consumer application like this, where you're compromising on basically every aspect in order to fit the form-factor, the losses from a swappable blade would almost entirely negate the benefits of the system.

In short, to have even the moderately functional product they've got, there's no way for them to have swappable blades. The system just won't be powerful enough to compensate for the losses.