I think Apple could have made a reasonably successful play at volume with cheaper Macintosh computers (or even licensing*) somewhere in the 2005-2012 time-frame.
Whether it would have been worth focusing resources on Mac over iPhone/iOS at this period (or more pertinently if Apple even had sufficient time and staff) is a different question. I personally doubt the gains from desktop (even big gains like 30-50% share of new PCs sold) would have been able to offset potential gains on mobile given a similar allocation of resources.
It is easier to see with hindsight, but the opportunity was clearly there for disruption of the PC market. This period coincided with consumer disgruntlement with Windows Vista (2007) and then Windows 8 (2012) along with the XP upgrade cycle (2007-2014). If deciding to license Apple could have capitalised on OEMs disgruntlement as well with the Microsoft over Surface (2012-present).
There was certainly some unique opportunities to capitalise (mostly missteps by Steve Ballmer's Microsoft). With hindsight it is easy to see how Microsoft's ability to respond, the (relative) competitiveness and desirability of Windows were overestimated by most analysts at the time whilst simultaneously the (relative) quality of OS X as a desktop platform was underestimated.
There seems very little point now as overall the market is barely growing and there are fewer obvious opportunities for disruption. In fact I think the far more likely outcome next (and threat to Apple) is OS X is disrupted by the web platform.
* Search for Steve Jobs Sony Vaio - it was a consideration at one point.
Jobs liked the design language of the Vaio at the time. If Apple had played their cards right they could just have invited a lot of disgruntled Windows users over. The Mini was originally a part of that strategy but I have the feeling that Apple's 'A-Team' can only work on a limited number o projects at once and there's been plenty of evidence over the years where certain products have been released to big fanfare then been left to rot and fall behind the competition because Apple's small team of tech wizards have moved on to the next big thing.
Just look at some of the products that have fallen by the wayside: the Airport range, Thunderbolt displays, and Aperture software are but just a few of the known things that have fallen by the wayside. In addition, the macOS team has been plundered to get iOS working well on time yet bugs have proliferated and the Mac App Store is a pale shadow of the iOS store.
Perhaps this year will be another 'Snow Leopard' year with Apple dazzling us with tangible improvements - more cores, perhaps new designs, and maybe even a price cut in parts of the world. There's been few leaks of software innovations so maybe it's right that this year's WWDC could be hardware heavy and a little light on the software innovations?
Intel charge serious big bucks for their CPUs - Apple could produce Ax family CPUs at a fraction of the cost, and that cost saving allows the use of all-SSD for storage across their iOS devices rather than a real life price cut. Consider how much the iPad Pro costs compared to a MacBook and the relative benchmarks for each. They are getting very close now especially since Ax CPUs are getting double digit performance increases annually whereas Intel have hit the proverbial brick wall and are starting to throw cores at the problem in a bid to stave off competition from AMD.
Licensing macOS was a mistake as all that did was create a race to the bottom which Apple couldn't compete with. And they are the ones who are meant to be innovating. Remember that Apple 'charge' for macOS by being the only Mac makers in town - that's clearly part of the reason for the steep 'Mac tax' on prices.
I don't have any major issues with Apple pricing hardware and making software cheaper. It's better than cutting the throat of hardware margins and hoping to get software sales. The Mac App store - another missed opportunity - is only slowly becoming the place where Apple makes a tidy profit from their percentage of App Store downloads.
If Apple made a headless machine that interested eSports players (high speed cores, decent graphics, fast storage, and portable so you can take it to LAN parties - I'm thinking stuff like Overwatch for example) they might then catch some the games playing people, scoop up budget video editors and photo shoppers, and perhaps even an almost lost generation of tinkerers. But their sales figures for the Mini can't lie - as a machine it's been static for years.
During that time they decided that GPU compute was the way ahead - having already tripped themselves up with a Mac Pro that didn't interest enough potential Mini upgraders (priced very high). If the Mini comes as a headless machine with just integrated graphics - bring your own eGPU - and a couple of Thunderbolt 3 sockets then that certainly might interest some of us, even with an i5-8250U CPU. Certainly we Mini users would have to think long and hard IF there's a replacement product for the Mini soon. If we ignore it, there's every chance that Apple will axe it in due course.
But there's every chance that the Mini gets delayed deliberately for a simultaneous unveiling with next year's modular Mac Pro. Heck, as said in other threads - the base Mac Mini might even BE an SKU of next year's Modular Mac Pro.
Something like that would certainly be better than the shock of finding the 2014 Mini lost 2 cores from the upper SKU and had the RAM soldered down to prevent cut price upgrades. It was already slightly disappointing to find no quad core in the 2014 Mini, after finding out that upgraded models would cost that much more because RAM and SSD had to be bought direct from Apple at the time of purchase AND THEN have the whole machine get neglected for over 3.5 years is incredibly sad.
Having said that, for me, the potential addition of Thunderbolt 3 makes it possible for users to buy a useful machine which could be expanded with GPU, high speed SSD storage etc.