Actually, if you listened to the criticisms, it's wasn't that Apple didn't offer a headless Mac. It was that Apple didn't offer one where you could upgrade the internals yourself. The 5k iMac was frankly good enough for "serious" work and the display wasn't all that bad.
That's part of it - and probably what the loudest noises from the peanut gallery were yelling (even if they had no interest in buying one) - but its not
just that - the whole all-in-one thing is questionable from a "pro"/serious enthusiast perspective.
That Apple 5k@27" panel is certainly a great display -
if you want a 5k 27" display permanently welded to your computer. If you wanted some other size/shape/technology/configuration - tough.
Want a pair/trio of displays with the same size/height/resolution/colour profile? Tough - closest thing would have been the LG Ultrafine - not necessarily an exact match.
Want a 32"+ display? Tough.
Want a display on an articulated arm... well, you could get the VESA option and put the whole Mac (with all the extra bulk and sockets for your network & external devices) on an arm. Kludge.
Want a 4k/1440p/1080p display (maybe with a particular colour profile) because its actually a better fit for your workflow? Tough.
...OK you can always have those things as a secondary screen, but then you've got that big 27" black mirror that you don't really need.
Then there's the whole thing of having to trash the screen along with the computer (whichever you upgrade first).
I bought a high-spec 5k iMac i7 in 2017
only because there was no "headless" Mac with comparable CPU/GPU power - I can't say that I hated the screen - its pretty good - but it isn't what I'd have chosen if a suitable headless machine was available - I wanted a matching pair of 4k displays. The headless options at the time were the Trashcan (by then, obsolete tech that even Apple had admitted was a dead end) or the 2014 Mini (Apple ditched all the more powerful Mini options in 2014). I did seriously consider the Trashcan or waiting for the already-announced iMac Pro, but the former was just too old, while the iMac Pro was more that I wanted to pay for a
compromise.
I've now moved to a Mac Studio and a matched pair of 4k+ 28.5" 3:2 displays*. Sure, they'd lose in a side-by-side "quality" comparison with my old iMac screen - but they're pretty good. Not being a YouTube influencer, I didn't buy them to do A/B comparisons with Apple displays, I bought them to
use - and the combination of resolution, size and having a matched pair gives them far greater
utility value to me than a 27" 5k.
(* which the world could use more of - plan B would be regular 27" 4ks).
It's not that Apple isn't offering them what they wanted. It's that they don't want to pay more than the absolute bare minimum for it.
What a lot of people wanted at the time was a tower system with PCIe slots and space for internal drives, to replace the "classic" Mac Pro (2006-2012 RIP) for around $2000-$3000. Apple
didn't offer anything like that after 2012 - and even after they acknowledged that in their spring 2017 "apology" their response was the grotesquely over-engineered 2019 Mac Pro which started at double that price and didn't start to make price/performance sense until you spent $12k+ on it.
Post Apple Silicon, a regular "tower" Mac for a sensible price isn't going to happen. The new Mac Pro is even more niche than the 2019 and only those who absolutely need huge internal SSDs and/or specialist interface cards for which TB4 just doesn't cut the mustard need bother with it. I wouldn't be surprised if the current Mac Pro is the last of its ilk.