Why don't intel desktop chips have hyperthreading lower down the pecking order? The whole MBP line chips have hyperthreading... IMO it would make more sense for a desktop to have hyperthreading with more thermal capacity.
It's probably a combination of cost, binning tiers, and marketing.
Given the throttling results though, it may have made sense to have non-HT 6-core i7 mobile chips.
Do you really think the i7 or i9 will not be desirable in the hypothetical 2018 iMac?
Well, I am noise sensitive, and I already found the i7-7700K undesirable because of the fan noise under load. I had one and then returned it to get the i5-7600.
The i7-8700K will run at least as hot, and sometimes hotter. An 8th generation i9 doesn't officially exist, but there is an i7-8086K which is faster than the i7-8700K and likely runs even hotter.
There is a new chip line coming, but that's actually 9th generation:
i9-9900K 8-core 16-thread
i7-9700K 6-core 12-thread
i5-9600K 6-core 6-thread
i5-9600
i5-9500
i5-9400
etc.
I'm sure the i9 is going to be very, very hot, and it would likely require a new cooling system in an iMac. The iMac Pro cooling system would be perfect for it though. Actually, it'd be great for the i7-8700K and i7-9700K too, and all of the other 6-core chips too for that matter.
BTW, there is a new Core i3-9100 coming too that's almost as fast as my Core i5-7600. That would make a decent entry level 27" iMac, but I'm guessing Apple won't use it, and I'm guessing they won't use the existing Core i3-8100 either.
Sure, the faster HyperThreaded chips would be great performance-wise for Logic Pro X, but in music production, noise can be an issue. That's why you see so many posts here asking which iMacs would be the best balance between noise and performance. It seems most agree that the quad HT i7-7700K is good enough for even elite users with many working fine with the quad i5 chips too. The i5-8400 is in the performance ballpark of the i7-7700K but usually runs cooler than the i7-7700K, which makes it a good choice.
tl;dr:
I'm thinking the best balance for music production in fall 2018 may be the Core i5-8400. 6-core non-HT performance which is close to the i7-7700K, but usually with lower power usage.