But, imagine if that knife also vibrated at an ultrasonic resonance!

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.

Anything's more fun with a knife involved.

If you're applying force downward, you're not using the knife right anyway.

I mean there are some situations where you want to chop, but for that you want a knife with a different shape than this. A straight, flat edge that's full length can be in full contact with the board at one time.

The shape of knife in the picture is for slicing. Light downward pressure while pulling smoothly backwards. If you do it right, you should be able to slice a tomato without smashing it.