Here's my two cents on iMacs and general longevity of Mac hardware.
I'm a professional photographer working with 35mm digital and medium format files (100Mb+ transfers, imports and multiple (10-50?) 16-bit Photoshop TIFF edits of images regularly between 1-2Gb alone. That's just for a single image.
50-150Mb raw files are stored usually on external SSDs over USB 3.0, Thunderbolt 2 and recently USB-C and Thunderbolt 3. There is a mass of data constantly streaming between the 5-6 computers in my studio, Dropbox eats up at least 20% of every computer's CPU almost 24 hours a day, 7 days a week, MacBooks are connected to external displays, sometimes two, we're rendering contact sheets, compressing, reading and writing to hard drives almost all of the time. Year in, year out for almost 5 years now. Our computers are being used in every sense of the word.
I run SMCFanControl and iStatMini on all my Apple hardware to tinker fan speeds and keep an eye on temps. I think I thrash my hardware pretty hard - so if I can make use of this, I think most of you should as well. Making it 3 years on a top of the line Apple hardware and then offloading? Poor judgement and / or your buying into marketing bs.
I've been getting 7 years out of my Apple hardware. Granted, my 2010 27" iMac (which I only just offloaded two of this week) was upgraded halfway in to 20Gb RAM and a 256Gb SSD - this isn't an option anymore so I'm fully behind the theory to buy the best you can (afford), ignore the marketing hype and focus on maximising the value of your hardware.
I was running Sierra 10.12.6, Photoshop CC 18.1.2 and Lightroom 6.12 up to a week ago on my iMac and the only thing that got in the way for me was USB 2.0 - that's why it had to go. Once we started playing around with 1-2Gb exports, the writing was on the wall for our two studio iMacs from 2010. Don't let anyone convince you that you need to update every 2-3 years just because you're wanting to suddenly stream 4k Youtube videos (seriously?), encode H.265, do that light bit of editing you've always wanted to do or - bane of all - game on a Mac (ugh, get a PC or a console already - these are simply not gaming machines and never will be, everyone who is serious about gaming knows this.)
Anyone who thinks flipping their hardware every 1-2 years for the latest thing is either juvenile, isn't a professional using their hardware effectively or doesn't take themselves or their bank balance very seriously. Sure, you can get the latest toy but where does that really get you? You're buying into hype and The Next Great Thing That Is Never That Great To Begin With.
Am I happy with the way Apple is going with their hardware? Yes, I still think they make some of the most beautiful machines in the world. Could they be doing more for the pro market? Undoubtedly, that has slipped down the priority list for Cupertino. Are they focusing their attention on the slimmest, the smallest, the most locked-in - yes, but hell, if they weren't innovating in this way, I'd be worried. I know how long Apple hardware can last and its a lot longer than most may think.
Would I buy another iMac for professional work? Hell, yes. Do I think its better than a MP and a monitor of my choice? Nope, but thats only for the 5% of us out there who can / need to tell the difference.
Long live the iMac; boo, hiss the fanboys and marketing hype. Maximise value and don't buy into the hype. Your machine is fine, life is good, dissipate that obsession into something more worthy.