Here's a copy of my research to save you the trouble. Now that you mention it Apple hasn't ever skipped a CPU generation aside from the Broadwell/Skylake split release. Huh.
CPU . . . . . 3rd . . . .4th . . . . . . . . . 5th/6th . .7th
GPU . . . . . GT 6xx. . .GT 7xx/R9 M2xx. . . . R9 M3xx . .RX 5xx
$1099 21.5" . 03-2013 -> 06-2014 ------------> 10-2015 -> 06-2017 {15, 16, 20}
$1299 21.5" . 10-2012 -> 09-2013 ------------> 10-2015 -> 06-2017 {11, 25, 20}
$1499 21.5" . 10-2012 -> 09-2013 ------------> 10-2015 -> 06-2017 {11, 25, 20}
$1799 27" . . 10-2012 -> 09-2013 ------------> 10-2015 -> 06-2017 {11, 25, 20}
$1999 27" . . 10-2012 -> 09-2013 -> 05-2015 -> 10-2015 -> 06-2017 {11, 20, 5, 20}
$2299 27" . . 10-2012 -> 09-2013 -> 05-2015*-> 10-2015 -> 06-2017 {11, 20, 5, 20}
$2499 special . . . . . .10-2014 -> 05-2015* {7}
*Late 2014 5K price cut.
Average update interval: 16.1 months.
Median update interval: 20 months.
The numbers do look better if one instead starts at 2011 or earlier, but back then there were only 4-standard configs plus an educational model.
Anyway, I can think of two logical reasons we didn't see 6-core RX 500X iMacs along side the MBPs.
(a) Apple was waiting on 9th-gen CPUs for the 5K lineup and chose to delay the 4K update to keep releases in sync.
(b) AMD messed up this year and Apple is committed to waiting on RX 6xx GPUs coming 2019.
Assuming either is equally likely I'm guessing 50% chance of new iMacs on the 30th.
...although if you want to be super optimistic and believe pigs can fly there is also:
(c) Apple has a special anniversary iMac with an Nvidea RTX 2070 which is why Nvidea hasn't released Mojave drivers yet.
(Edited to clarify I don't think (c) is a logical reason and was included as an attempt to offset my prior pessimism)
Impressive bit of research, the unusual split release happened to include the (very much delayed) Broadwell Crystalwell CPU (including the only use of the Iris Graphics 6200, in desktop form as it happened) in the brand new 4k iMac. I thought at the time that Apple had decided to wait for that. As with the 15" MacBook Pro the 27" iMac stayed on Haswell CPUs way too long - relying on a (minor) graphics bump to lend it some freshness.
Apple were also in the process of switching non-5k monitors to 5k, a process which was later completed for the 27" and now we're about to see the 21.5" range go fully 4k.
Incidentally, with the 4k iMac Apple never went back to integrated graphics for driving their 4k monitor after the original 4k was superseded. They've always added a GPU - even if a MacBook Pro refugee in the case of the 4k iMac.
Responding to your two serious points, I have now had some time to sit down and compare the 9th generation to the 8h generation CPUs from other posts and news articles and conclude thus:
a. Apple should use i7-8700K for top of the range 2018 iMacs; do you think Apple really wanted to get an i9 into this year's iMac and delayed it till October for that reason? The 9th Generation CPUs
require a Z390 chipset which has only just been released but is quite pricey - Apple generally want to keep prices down so haven't gone for top of the range chipsets in years (since Sandy Bridge I think).
The 8th generation CPUs which have been out for a few months at this stage can get away with a (cheaper) Z370 which has been out much longer and in fact as far as Apple are concerned they could have saved a few dollars per unit by opting for the
B360 which has been out since around April - the same move that they have done for other iMac chipsets over the years.
They could have updated the job lot in June or July 2018 but some of the parts that Apple have been waiting for have been available since 4Q17 - the
Coffee Lake i5-8400 for example - while they've been waiting for the correct chipset and graphics to become available. But look closer and you see that the
i5-8600 (suitable for mid SKU 2018 iMac) didn't launch until April of this year (officially 2Q18) so there's been quite a long gap between first release (of the fastest and the cheapest) before the range was filled out.
Intel would have been able to provide the full Coffee Lake range to Apple for testing by around April 2018 with engineering samples before that - months after the first samplings of top i7 and low end i5 were on sale.
Even so, there's been lots of stories about extra heat generated from the 6 core CPUs that Apple will hopefully aware of, and they have solution for that already sat there and tested in an iMac Pro.
We don't know how long it would have taken AMD to prepare the much suspected 500X series GPUs (most pundits suggesting it's an twice warmed over/overclocked 400 series) but remember that the Macbook Pros didn't come out till July. If that wasn't for tactical marketing reasons then perhaps AMD were the slackers here.
What if the Apple marketing people needed something to headline an October event in case the MBA refresh didn't make it? The 2017 iMacs had already been launched in June of that year because of the late arrival of the Kaby Lake CPUs -
they weren't generally available till 1Q17. Annoyingly, that meant that in Europe the old 2015 iMacs got a
price increase in October 2016 with no spec bump due to currency exchange and BREXIT. The dollar price remained unchanged so your data remains consistent though.
And this year, we get Coffee Lake Refresh, as the 9th generation seems to be called now. As with the Coffee Lake the CPUs are being released spread across half a year - on reflection, do you think Apple would steal from next year's iMac CPU range, especially with the core count rising substantially?
To be fair, unlike with the 2014 Mac mini, Apple probably don't mind making 2 different iterations of iMac chipset within a range - they currently must be doing that to keep the non retina iMac going with a mobile CPU - but this is going to essentially create a 2 tier 27" range with presumably the top SKU a new beast altogether.
If the purpose of this is to allow Apple to choose pricier 9th generation K series CPUs - which have been released ahead of the rest of the series due in 1H 2019. The i9 would probably need a special cooling solution which would I fear Apple could inflict across the range to keep them as the same SKU. In effect they'll lock away the RAM access door. They've already tested it with the iMac Pro and the 21.5" had had it for years so I think it'll be inevitable for Apple to let the 2018 27" iMac follow the iMac Pro in looks if they go this way.
I'd expect it's more logical that Apple intend to refresh the 2019 range using all 9th generation CPUs at WWDC 2019 when it's the earliest they could introduce the Modular Mac Pro for immediate sale.
As we can see from looking over recent news articles, the i9 would be mightily impressive for that range but would compete too directly with the base iMac Pro. It could look nice in a headless Mac though.
I'd be more impressed if Apple chose to use the
Xeon E range in the iMac and effectively rebadge some or all the 27" iMacs as iMac Pros. They could choose from 4 core to 6 core, with or without hyperthreading, and perhaps even offer a VEGA for graphics as an option.
The 21.5" iMac could continue using non K series Core CPUs or perhaps even go their own way too.
b. AMD are once again warming over the same old graphics chipsets that have been around for a couple of years by adding an X to this year's 500 series iteration. They only have to offer the usual (big, exclusive) discount to Apple and they'd probably take it as NVIDIA are (as mentioned) essentially off the table and better graphics is even less of a priority this year for Apple than ever with the iMac Pro around - VEGA appears to be reserved for the iMac Pro and for segmentation purposes Apple wouldn't mess around with that distinction especially when the headline reason to get a 2018 iMac as to be extra cores in the Intel CPU.
I think it would be easier to characterise the 2019 iMac as the one with the graphics boost from using AMD Pro 6xx series next year. We don't know how the 9th generation Intel CPUs will pan out but it won't be as a big a leap as for the 8th generation seven if you take into account SKUs with 8 cores, no hyperthreading or the i9 with 8 cores, 16 threads. Those will almost certainly lock the RAM away for the entire range if not already with the 2018 iMacs.