It is disappointing to see how little you understand computer hardware.
Listing a bunch of meaningless low end CPU's doesn't make you right.
I didn't know iMacs are just mediocre computers with medicare level hardware waiting for the most mediocre refresh Intel is capable of releasing.
There are no holes man, all the low end CPU's you listed are trivial refreshes of the already existing 8 series CPUs.
Irrelevant. Vega GPUs are currently not high end anyway.
Saying that Apple decided that the Vega 56 and 64 are for the iMac Pro and this is the reason they can't use them for regular iMacs is just a random excuse that doesn't make sense.
Apparently, I understand PC hardware way better than you do as well as Apple hardware and how Apple decides when updated models will ship. Read and learn.
Apple currently uses the Core i5-7400, Core i5-7500, Core i5-7600 and Core i7-7700 (along with the Core i5-7600K and Core i7-7700K) in the 2017 iMac. If Apple's intent is to skip the 8000-Series CPUs altogether, then they are still waiting on Intel to announce and ship the equivalent 9000-Series successors, which has not happened. All Intel has shipped is the higher end unlocked K-Series, which Apple only uses in the top tier 27" iMac (i5-7600K) or as a BTO option (i7-7700K). Apple does not release CPU updates piecemeal as Dell, HP, Lenovo, et al. do. While you view them as trivial refreshes, Apple will still use these parts only once Intel announces and releases them, even if that means holding up the entire iMac product line. The same thing happened with the Mid-2017 iMacs.
For Apple, this is as much a business decision as it is a technology one. Intel's chip supply issues are still a concern and they aren't expected to ease until mid-2019, so I expect that Apple is remaining cautious while Intel catches up with demand, and continues to fill out its CPU portfolio.
I can see Apple using the 8000-Series in the 21.5" iMac as the supply situation seems much more stable, while keeping the 9000-Series for the 27" iMac. They did this with the Late 2015 21.5" (Broadwell) and 27" (Skylake) iMacs.
The Vega 56 and 64 may not be high end any more, but they are the only ones shipping now, as the Radeon VII still has about a week before release and a GPU that expensive is not going into the regular iMac. If the Radeon VII is destined for the iMac Pro so be it, but that is not going to happen until Intel releases updated Xeon W CPUs based on the Basin Lake refresh X-Series.
The W-3175X just got pricing today ($2999), and is simply too hot for the iMac Pro chassis. However, the main reason why an iMac Pro update is a ways off is that there have been zero signs that updated Xeon W CPUs (W-3xxx) are on the horizon. Until that happens, there is no meaningful upgrade to offer users.
Right now, Apple is waiting on Intel to fill in the lower tiers of the 9000-Series, AMD to release 7nm Navi GPUs suitable for the regular iMac, Intel to release updated Xeon W-Series (up to 22 cores, if rumors are to be believed) and they will either use the Radeon VII (Vega Instinct) or something else that has not been released. Until those dominos fall, Apple is not going to update the iMac or the iMac Pro.
Whether you think it is some random excuse or not is completely irrelevant to the facts I have laid out.