I agree with the reasoning, and would like to see Apple continue to make aspirational hardware, but maybe the mainstream stuff is good enough?
Even Siracusa admits that - he's found it hard to articulate what a true "Mac Pro" would do that you can't do with other things.
Back in the heyday of the $100k Mac Pro you could certainly imagine it doing things that wouldn't be easily done by anything under $50k, and it would look good doing it.