Apple didn't update the Mini because the difference in a desktop environment between Ivy Bridge and Haswell is almost nothing. Haswell (and Broadwell for that matter) are more about pushing the GPU and reducing power usage. Yes the GPU would be nice, but the power usage is next to worthless on a desktop.
We would all love to get an Iris Pro GPU in a Mini, but that's about the only real advantage of Haswell. Even the HD4600 hasn't proven to be much of an advantage over the HD4000 so it really takes the HD5000 or better to be a true "upgrade" for a desktop.
What I want from the next mini: Dual Thunderbolt ports. Let me run both my Displayport monitors on my Mini at the same time at full resolution without a USB 3.0 Display driver or some kludgey external GPU setup.
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I posted that Apple wouldn't release the Mini prior to shipping the new Mac Pro -- for fear of impacting sales of the Pro model.
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You really think that a Haswell mini that would get maybe 500 points more on Geekbench and a marginally more powerful GPU would somehow eat away at the new Mac Pro with it's Dual GPUs, able to run up to a 12 core processor, power something like 6 monitors, 64GB of RAM, and Multiple Thunderbolt 2 ports?
Even if they went to the Iris Pro GPU but kept the same form factor, you aren't going to pull a Professional from the Mac Pro to the Mac Mini. There is no way no how. Without the ability to natively run multiple 1440P monitors, that a Professional is going to go the route of a Mac Mini. The iMac, with its ability to use as fast of a 4 core processor as the Base Pro, and run 3 1440P monitors (include its built in display) is the real competitor to the Base Mac Pro.