it only has to be refreshed every 2 or 3 yrs or so to be relevant but would sell by the bucketload
Yes, but Apple doesn't like to sell computers by the bucketload if it can sell other computers by the shipping container-load.
1. The whole market trend is towards laptops rather than desktop systems. That's one bite out of the iMac market.
2. Intel iMacs usually had more powerful, higher-TDP CPUs and GPUs than comparable MacBooks. Post-Apple Silicon, they're running the same chips (until you get to the Mx Ultra). Many people who previously had a MacBook for mobility plus an iMac for heavy lifting can now just use a dockable MBP for everything. Another bite.
3. The 24" Mx iMac has a substantially bigger, better screen and, relatively speaking, better CPU+GPU than the preceding low-end 21.5" iMac. That's going to poach some low-end 27" iMac customers. Munch, munch...
4. Apple now offers a
far better, up-to-date range of "headless" desktops - the Mx Mini, Mx Pro Mini, Mx Max Studio and Mx Ultra Studio (when I last bought a 27" iMac in 2017, the alternatives were the 2014 Worst Mini Ever or the 2013 Trashcan... 'nuff said). Guess what: not everybody likes all-in-ones when they're given a choice. Chomp chomp!
5. I'm not a great fan of the Studio Display on the value-for-money front - but it
does offer a near-iMac experience at 5k and, by all accounts, is a very nice display. $4k gets you a better-than-base Studio Max and a Studio Display - cheaper than the $5000 base iMac Pro, and in the same ball park as a comparably tricked-out 2020 iMac. Plus... if you'd bought a Studio Max + Studio Display back in 2022 and now fancied an M4, you'd only need to replace the computer & keep the display.
The $1800 5k iMac was always a bargain and, yeah, a modern Mini + Studio Display combo would set you back $2200, so that's not so great. I agree we've lost that - but today there probably wouldn't be the volume to make that possible. Also, I suspect Apple bet the farm on 5k panel costs dropping as the wider PC market adopted them - something that has never really happened because 5k isn't such a big deal for Windows users.