I think we all know that Haswell was an after thought after Broadwell was pushed back so many times by Intel.
The present enclosure is such an old design that Apple will, and probably already has a new design to fit in with Apple aesthetics so as not to be embarrassed with a 15 second blip at an event.
Maybe by next October they will introduce a Broadwell/Skylake Mac Mini that will bring the Mini into the future and will add the "wow" factor back into our little friend.
The Haswell refresh, as many people refer to it, is indeed Intel's placeholder for the delayed Broadwell. Broadwell itself is due to be superseded by Skylake which is the one to look out for as far as Apple are concerned because it will bring Thunderbolt 3 and with it the ability to connect to a 4 or 5k display with a single Thunderbolt cable (no messing about with drivers or dual cables etc).
Current timetables suggest Skylake will appear in late 2015 but given the Broadwell has been delayed I am not hopeful of it arriving on time - it would give Broadwell a very short shelf life as far as Apple are concerned.
2 years for a Mini update seems to reflect what happened to the Mac Pro which went 2 years between 2010 and 2012 between updates only because the CPUs in the 2010 were about to be EOL (in other words Apple had to refresh or otherwise discontinue due to lack of parts, like with the
iPod Classic).
Here's what I think will happen next year:
Retina Macbook Air arrives, killing off the last remaining Ivy Bridge Macbook Pro still in the Apple Store. The Macbook Air with Retina will sit alongside the the non retina Air for one generation. Apple can remove all Ivy Bridge era components from supply chain for new builds.
Retina 4k iMac arrives, with i7 and improved graphics, possibly AMD, possibly the much mooted Nvidia 970 (more power efficient and running much cooler for the smaller case) that's appeared in Yosemite builds.
Non retina iMacs would probably get the refresh too if only to get a Thunderbolt 2 interface. They skipped the Haswell refresh chips in return for a small price cut on most if not all models not long ago.
These products will be Broadwell and should carry the Thunderbolt 2 interface.
The iMac with MBA innards from the last update seems to have pointed the way forward for the Mini this time and it's going to be worth looking at what happens to the non retina iMac going forward.
I've not done enough research to determine if Skylake mobile sockets are going to be compatible between dual and quad core (as they were with Ivy Bridge). That seems to be the main factor that has generated the decision to go dual only across the board and created all of the hubbub about it. It would appear to be logical to monitor Intel's Skylake news on hardware websites to help us predict what may be coming.
For what it's worth, if a Mini is coming, I'd expect it to be a Thunderbolt 3 equipped Skylake refresh sometime in the first half of 2016 and to arrive at the same time as a refreshed Thunderbolt Display (with Retina!).