The mac pro isn't exactly in a vacuum. It is its own category, but there was always some competition between the bottom mac pro and top imac assuming specialized peripherals were not required. You're envisioning the 17" returning as something they've never made. Apple has a high markup, so they don't go to such extreme configurations due to where that would leave the price at their normal markups. They have also never engineered a logic board to take 4 sodimms, which is what would be required for 32GB at this point. They would have to grant it a larger power source for that kind of gpu. This is just really far from anything likely, although I would still want one. I don't care that much about the displays. None of them compare to what you can get in a desktop display. I'll start paying attention when they're close to comparable in quality.
It probably made up enough volume that they decided to be cautious. The 13" itself increased in price on at least one occasion though.