Apple make VG profit margins on their iMac range, so I agree they're highly unlikely to offer anything that might risk cannibalizing those sales.
Apple makes VG profit margins on all of the Macs. There are no "loss leader" Macs which are dependent upon sales of some other Mac to offset lower than nominal (for Apple) profits on that product.
So users moving down to lower priced products because their workload has pragmatically plateaued and now only really need a lower product is just fine. If the workload has plateaued so much can transition to an iPad. That is OK too.
What Apple isn't going to do is the fraticide flavor of cannibliziation. Where products priced about the same have very approximate peformance/functionality.
If just swapping mini sales for iMac sales or vice versa all that does is just rearrange the deck chairs. It isn't growing the overall Mac market. Rearrangng the deck chairs can't be the primary objective or result.
Most of these "add stuff to the mini so it is equivalent to iMac" tracks basically kill off the mini's margin. What you are saving is not only engage in fraticide, but take less money. It shouldn't be surprising why probably isn't going to go with that strategy. That's pretty much what was done in the early-mid 90's. It was giant bust of a strategy.
That just about rules out a more powerful Mini anytime soon or a cheaper, upgradable Mac Pro, when that line finally gets updated sometime this year.
The dubious assumption here is that the iMacs can't also be moved forward at the same time. There should be tweaked mobile offerings from both AMD and Nvidia by Q3-Q4 when the updated iMac should be do to roll out.
If Apple upgrades the iMac there is nothing in Apple's strategy against moving the mini up into that vacated performance zone.
The likely problem with the top end GT3 Haswell solution is that it is likely going to cost more. If Intel is bundling the RAM into the same CPU package as the GT3 and selling that bundle they are certainly going to charge a premium for that. Intel doesn't give away "free" any more than Apple does. Apple isn't going to eat that increase resulting in lower margins. So it is questionable that top end GT3 will make the Mini's price point constraints. I suspect there are some TDP constraints too for top end GT3.
There is a range of GT3 HD5200 and HD5100 coming. For the mini's price point it will likely be the more limited one that probably doesn't put an updated, but price equivalent, to a GT640M in danger.
The iMac is higher priced so it can afford better parts with no margin loss.