"A few power users will complain, but most won’t care: by that time, most former Mac Pro customers will have already switched away."

This is a statement that is general and without any basis at all. Where does the OP come off writing "most former Mac Pro customers" I can almost guarantee that this is simply pure conjecture with nothing to back it up.

"There’s even less of a reason to buy the Mac Pro today than I expected."

I run 3 monitors off a Mac Pro (24/30/24). I don't believe there is any way to do that on anything but a Mac Pro. I could run six with the two video cards I have now but can't fit them on my desktop. And I'm not using it for video editing or anything like that. I just like to work with as much screen space as possible.