The thing is a lot of people are also too focused upon CPU performance. For example, a friend a mine who does web design for billion dollar multinational companies uses a 2014 iMac, and for his work the performance is more than sufficient. He does want to upgrade, but not because of CPU performance. He just wants a new form factor.You make a good point re: mature INtel iMacs vs '1st mover' AS iMac. One is the end and mature. The other is the beginning and immature.
The both have their pros and cons.
I do think the performance of AS is going to surprise many, though...relative to how great the A12z performs for it's target market. If the AS performs as well (relative to...) I'd expect more than '20%'. And I think that's not just down to 'raw' speed (though some of that will be the case...) but efficiencies of the software stack and the modular co-processor SoC approach. ie. It will be less about Geekbench and more about the overall user experience and certain workflows that could be greatly enhanced if the 'preview' of AS is referenced by the iPad and A12z.
But there will be no shame in a 27 inch Intel iMac with 10 cores and a a 5700 XT (even if it is a year old...) and it will run Intel apps decently and Bootcamp (for those that need it.) A new design would sweeten the deal, though. The new AS iMac shining it's bright light will badly age the Intel iMac boat anchored with dated design.
Workflow is very important yes, but I think in the initial few years the Intel Macs will have the workflow advantage in most scenarios, precisely because workflow is often not so much related to RAW speed. If it ain't broke, don't fix it. This will change as time goes on, of course, but 2020 and 2021 are going to be rough from the workflow perspective on a lot of people going with a new Arm Mac. Apple understands this and that's why Apple is probably releasing new Intel Macs shortly.
That probable 10-core iMac is going to sound like a vacuum cleaner though if it retains the same fan system as the current iMacs. In fact, I owned the 2017 iMac Core i7 for about a week, but hated the noise if I did anything CPU intensive, so I returned it for an i5 and am much happier. I saved a few bucks in the process too. I would expect the high end Arm Macs will have better thermals. If not, I will be disappointed.