... Apple is not putting a whole bunch of resources into redesigning their lowest profit/unit machine.
The mini is not so much low profit as relatively low unit volume. The margins are likely the same. They definitely make money on them. They don't sell as many. A contributor to that it needs more value for what limited space available. They need to invest something to keep from shrinking to even small number of units sold.
They partially offset the low units by reusing a high fraction of components and design solutions from the far more higher volume laptop solutions.
Nor should they just as most other businesses wouldn't. My expectation is the next mini will look like the current but with revised innards largely ported over from the rMBP (sans SSD).
The SSD would be relatively easy to include if starting from the the rMBP 13" motherboard. Just dump the lower, 2nd, and harder to get to, general standard SATA drive bay for the same PCIe SSD socket used on the rMBP. They get volume discounts by using the same parts. Instead of cutting motherboard short to open space for the bay, just keep the increased board space. Still can do Fusion drive (in fact, just like the iMac does it with 'blade' SSDs). Like the iMac, they don't have to put the SSD in the default configs ( so the minor change logical board change is actually hook up a SATA lane from the chipset as in the iMac ).
The only thing they 'loose' with that is server models with mirrored RAID 1 HDDs. I don't think that is a big deal. Folks who used two HDDs to go RAID 0 would be nudged to the SSD. Folks who need lots of bulk storage (more than a 2.5" drive worth) would be nudged outside (just like iMac and Mac Pro and every other Mac at this point.).
I think the fear is that Apple will go extra, super-duper cheap and not add the so-DIMM sockets ( and Ethernet and additional USB sockets ) back to the derivative logic board. That they will just mutate the Mini's enclosure to wrap around the rMBP logic board and they will be exactly the same board sold in different containers. The problem with that fear is that is precisely what Apple has
not done with the mini for its entire existence. There is enough volume (and certainly profits) to justify limited resources to tweak the board to fit the different role the mini has to play from the laptop.
IMHO, it would be better if Apple kept around a thicker, 0.8-0.9" , laptop that could carry a HDD and PCIe SSD [ all the while continuing the jihad against ODD ]. Fusion drive setups in the laptop space make sense for very similar reasons they make sense in the desktop space. That would make keeping the Mini's so-DIMM and HDD capable design simpler also. Between the combination of those two, the unit volume is likely going to be around as high as any of the individual slimmed down laptop models. A revised MBP 13" and Mac mini showing up together in 2014 ( or spread a month or so apart in 2014 ) would not be surprising.