That’s the funny thing. Many people asking for Apple to update Macs more frequently actually are still using Macs from 5 years old or even older. So why would Apple do what these people demanded when they themselves are not frequent upgraders?
Plus, unlike iDevices where Apple control the progression of the Ax chips, Apple relies on intel for the Mac. And intel themselves have gone to tick-tick-tock cycle, making every generation less and less of an improvement over the previous gen. I’m sure Apple is also tired of waiting for intel to deploy any new tech that they wanted.
You have a partial point here, I am in the camp where my teams updating frequency has fallen off massively in the Mac sphere. I used to update my PCs (self builds) annually. When I switched to Mac in 2006/7, Apple updated their hardware in component terms twice per year, hence the 'early' and 'late' year designations. I tended to update my Macs on 2-3 yearly basis. So 2006, 2008, 2009, 2011, 2012...and I adjusted my purchase type.
As a team, we recently retired 2 x 2006 17 inch MBPs, a 2009 MBA, 2 x 2009 iMacs, 2 x 2006 Mac Pros and a 2008 Mac Pro. We are still running 3 x 2012 Mac minis, 1 x 2009 Mac Pro, 2 x 2011 iMacs, 1 x 2012 iMac, 2 x 2011 11inch MBAs, 1 x 2012 iMac, 2 x 13inch MBPs and 2 x 2015 MBPs. We purchased these sequentially as part of a team. I have also had a 2015 MB but gave that to my Father as it was too slow, port constrained and had an awful keyboard....crap compared to the old MBAs in terms of ease of use.
Why have we not upgraded the old kit?
Well the 2012 Mac minis did receive a backward revision in 2014 (basically putting the MBA inside the MM case), so the 2012 model is the only quad core CPU, i.e. there has been no similar or better upgrade for that function subsequently.
The 2012 iMac was an upgrade for one 2011 iMac (thin version replacing fat version) but we kept the 2011 as a backup. Why haven't we replaced the 2012 iMac? Well this is when the speed of updates started faltering and there was little iterative improvement in the CPU/GPU combination until the 5k iMac - which we almost did buy but we couldn't get a matching second monitors as the graphical connector standards were not up to it. So we invested in my 2015 MBPs as a mobile workstation instead, these can run 2 x 4K external monitors and their own displays at the same time, i.e. they are more flexible than the then 5K iMac.
We haven't updated either the 2013 MBPs or the 2015 MBPs because of the keyboard issue. We were planning to replace the 2013 MBPs in 2015/16 but held off for the MBP update that was pending and when it got the MB keyboard we refused to upgrade (crap keyboards). This was why we plumped for the end of line 2015 MBPs, get the best keyboard ever while we could (a further reason, in addition to the 5K iMac problem). These MBPs also have the ports we use regularly. The current MBP line up just isn't good enough and would be dongle-tastic.
Why haven't we changed the 2011 MBAs? Well they killed the 11 inch models (which you could use on an airline economy seat tray table with ease). We tried the MB and it was rubbish, so that was out. These fit a niche below the MBP. Finally, the current MBA got a speed bump the other year of 200MHz (because Intel stopped making the lower speed Broadwell) but this was not a real upgrade, the actual last proper MBA update was 2014/5. At that time we were buying the other kit. We have not updated them since because it pains us to spend so much on such old tech, so we have not bothered and TBH the performance differences between 2011 and 2015 are minimal, plus the 11 inch is no longer available.
The 2009 Mac Pro is our oldest functioning device. This is a remote server. We could have got the 2010 or 2012, which we probably should have done. The updates appeared minimal and we had other priorities at the time. By the time we were ready we knew that the 2013 Mac Pro was rumoured...and that ended up being a disappointment as it didn't have the internal flexibilities of the cheese grater. So we got the Mac minis instead!
Meanwhile in iOS terms I personally have had the iPod Touch (2008), iPod Touch (2010), iPad 2 (2011), iPad 4 (2012), iPad Mini (2013), iPad Air 2 x 2 (2014), iPad Mini 3 (2014), iPad Pro 9.7 (2016), iPad Pro 10.5 (2017). For the iPhone, I have had the iPhone 5 (2012), iPhone 5s (2013), iPhone 6s and 6s Plus (2015), iPhone SE (2016), iPhone X (2017).
Why have I bought iOS items almost on an annual basis? Well the iterations are so fast that the gains are noticeable.
Quietly clearly we buy where the speed of innovation lies. We have not purchased recent Macs because 1) they are the same as the old kit, and 2) they are downgrades in terms of function if you buy the newer designs. We are now getting to the point where we need to consider if we switch from Mac, and that hurts.
Clearly Apple innovation effort is in the iOS domain. This is not all Apple's 'fault' as Intel has not bathed itself in glory but Apple has not bothered to even keep up with Intel iterations for their old designs in any real sense either, nor have they corrected poor design decisions affecting new products in the Mac arena, which wouldn't take much effort to correct.
The Mac feels dead and probably will continue to be so until Apple start their desktop/mobile ARM strategy and then, despite what they say, iOS and macOS boundaries will blur.