Obviously, not everybody has a great display, but most current desktop users I know have a very good (and *big*, usually bigger than 21 inch) display. Being forced to buy an "all in one" looks nonsense when you already have the display you need. This wasn't true in the past, because we were in the transition from CRTs or from small LCDs, but that's not the market reality anymore.
People who don't want an "all in one" don't want a Mini, because the Mini is less powerful in GPU. And don't want a Xeon either.
If Apple released a powerful i7 with a 2GB GPU, with 512 GB SSD and without display, they could offer it in the $1900 price range. And I'm confident this would sell in larger amounts than the whole desktop line in this moment, because that's exactly the kind of performance most desktop users expect in this moment. And Apple could meet such price within its usual pricing, without any new pricing policy
Let me get this straight: most Mac consumers are power users that already have displays and are willing to pay $1900 for a Mac Pro (whatever) instead of $1400 for an all-in-one.