Okay,
That was easy with computer, where you can have 3 windows open side by side, wouldn't be easy with ipad, which I mostly use to follow this thread.
I have thought that apple has used old chips in minis, like something already used in last years macs, but no, they were all launched in the same year than mini. They just got older after minis were launched...
So, this might be a way for apple to reduce the costs with new mini models; release new mini every 3 years, but use one year old cpu's!
Fat profits are here again!
We don't actually know what kind of deal Apple strikes with Intel though. The price for the CPUs on the Intel website are for tray volumes of 1000 at a time - obviously Apple buy more than that so will get a bigger discount depending on their rep - possibly to the tune of 25-35% perhaps but I have no hard evidence for scale of discount.
There may be discounts for yield as Intel get better at making older CPUs (ie they get fewer faulty ones per 1000 made) but I don't think they would strictly speaking discount for the age of the CPU unless Apple strike a deal for prices over time. Remember the likes of Dell and HP sell 'older' CPUs in their value lines.
And at some point Intel will deprecate old CPUs, less quickly for their mobile ones by the looks of it but it will happen in due course with the Haswell mobile CPUs which will ultimately decide the fate of the Mini if nothing else happens first.
It's true that today people aren't buying devices annually and could keep our computers for several years at a time but we don't all buy at the same time. The phone market expect new devices at least annually which is why Apple have to get on that treadmill with the iPhone. They have occasionally missed update years with the iPad for various reasons without harming sales too much but they do keep older devices like the iPad Mini 4 but crucially they REDUCE THE PRICES or in the latest incarnation BUMP THE STORAGE on an annual basis.
The iPad Mini 4 came out in September 2015 with 16Gb and 64Gb options. A 128Gb option was later added, the storage was doubled to 32Gb and 128Gb and latterly you can only buy a 128Gb version. Obviously you can get discounts through third party retailers or the Apple refurb store - and add cellular SKUs too. All of these gave buyers later down the line an impetus to buy at refresh time but the base technology has stayed the same even if the prices have changed slightly (it'll have been hit with the 2016 BREXIT product price increase in the UK but I can't specifically recall what that meant apart from a 20% rounded increase).
They have done none of these things with the Mac Mini (apart from the 2016 price bump) but I would have considered a purchase in later years if the base model had its storage bumped to 8Gb as standard, and then the hard drive was bumped to 1Tb (with 1Tb and 2Tb Fusion drives as standard higher up the range for example). Apple wouldn't have had to keep the price static either if they couldn't swallow the price of the upgrade but they have this mantra of keeping the price the same throughout the life of the product - only changing for local currency variations. This works great for keeping the perceived value up and internally they accept a lower margin at the start of a product line and end up earning their profit by the end of the product lifespan after the parts manufacturers have reduced the price of their products over time. Usually for iMacs this is an annual cycle but it seems at Apple nobody wanted to track annual SKU tweaks of the Mini since 2014.
What car manufacturers do - something that Apple were aspiring to be - is add more optional extras as standard to later model years and increase the price by roughly the cost of that option due to inflation. It gives the buyers a tangible difference between this year's model and last year's model to boost the impulse to buy. If Apple were selling a car i'd be interested if they sold the same model for four years on the trot and kept the price exactly the same year on year except for currency variations - Luca Maestri might not be so pleased though

.
It doesn't work the same for computers where the expectation is that prices drop for the same gear month to month until it's discontinued. Apple could have worked out a curve where base SKU specs got massaged to give buyers an annual predictable reason to buy. For most of us buyers that means a new Intel CPU and system - we might have accepted annual storage bumps but doing nothing for close to 4 years is the whole reason for this thread being in existence!
Looking further into the future, for the sake of example (figures guesstimated) the Intel CPU that goes into the Mini costs them $200, yet the A11 CPU that goes into the iPhone may cost them $20 - and the performance figures aren't too different when you compare benchmarks of the A11 with the 15w dual core Intel CPUs. This is where the juicy rumour of ARM CPUs coming in to Macs comes about. For the Mac Mini, an ARM CPU would enable Apple to use exclusively Flash NAND storage (like an iPad) and subsequently reduce the form factor because they no longer have to cater for a physical storage unit like a 2.5" hard drive and could design for less cooling due to lower overall TDP as well. I would also expect something like Gatekeeper to be used to make force ARM Macs to use an ARM Mac App Store - and the secret Marzipan plan appears to allow for a simple recompile for most app makers to get their app running on ARM.
If Apple were to decide to go that way with an ARM Mini they would decide on a reasonably high spec which would last 4 years - A12X for example would be out for iPad Pros by WWDC 2019 - with software locked down to the Mac App Store. Third party retailers would be allowed to run promotional deals from time to time while Apple would spec bump through occasional internal storage increases during this time.