He doesn’t really give a good use case for this. If it’s almost always docked, then how is its function different from the 16” MBP—other than in negative ways since it can’t run the same pro applications, at sustained speeds, many processes going at the same time, etc.? It wouldn’t even be much cheaper. The main advantage would have to be when it’s undocked, which he explicitly said would not be often.
The one possibly significant—albeit still niche—market for this device that I can see would be serious/professional digital artists who want a 16” portable canvas to use with the Apple Pencil. But of course the big caveat is the software they use has to be available on iPad (I’m a digital artist and my software is not).
Other than serious artists, even in this iPad enthusiast forum I think only a small minority would actually purchase this, especially given the likely high price. Imagine how small the market would be out in the real world.