It is not whether there is a gap. It is whether there is a gap which won't inflict significant cannibalization on Apple's other products that is the issue.
There is a gap plugger and then you dismiss it. That's twisted. This topic with continue to circle going nowhere until folks fess up on that point.
The primary significant differential is that the iMac has a monitor and this "monitorless" version does not. Mostly this really revolves around whether the monitor is bundled in the system or not.
If a significant number of folks buy a new apple monitor + headless what is the real difference between that and them buying an iMac ? None. All of those are 100% overlap in market to the iMac. Even more damaging is that they would likely overlap largely in price also. What you need to look for is huge gap in the product line up price points. There aren't any. ( in contrast to the persistant huge gap between Macbook and iPod Touch where the iPad landed. ). Note that even the "extra deluxe mini server" is still priced below an iMac. That's how rigid their pricing model goes.
So have old monitor want to continue to use and buy new Apple monitor so can do dual. Again, where is the difference from iMac ? You can go dual monitor (with second a non Apple one) with iMac.
OK so then folks don't buy any Apple monitor ( going Scrooge McDuck on less expensive new monitor(s) from someone else). Apple sells headless and gets $0.00 for monitor. Where is the win/win there? Apple looses. They really don't have high incentive to follow that option. They tie and bundle MacOS X so they have the slight upper hand here.
In the current setup, Apple doesn't have to compete much in the external monitor market. The bulk of their panels are locked in. That's helps them keep component prices down (predictable bulk buys) and system prices up.
Frankly, if Apple would increase the size of the mini (e.g., double the width, but no taller or deeper ) could stick a Core-i and discrete GPU in there and would cover most of what your "pro" version does without adding much overlap with iMac ( if priced and spec for separation. ). On next iteration of iMac when the core-i goes across could drop Core-i5 iMac and Core-i3 on "slightly" enlarged mini and still have separation.
For whatever reason folks are addict bug mug, coaster sized 2-dimension footprint of the Mini. That's the blocking factor here. Not overlapping new product line that overlaps with iMac on pricing.
