I think the fear is that this isn't going to be the latest and greatest merely the latest. 68k>PPC>Intel each new arch was an obvious step forward compared to the old. The rumors of going to arm? Well a lot of people including myself don't see it. I just don't think there will be any performance increase from the process.
I have to agree that in the higher end iMac and Mac Pro lines that professionals use, the performance would greatly suffer from whatever ARM based chip they could produce by then (2020). Here's what I think Apple may do to smooth out the transition to using their own ARM based processors throughout the Mac desktop lineup. Macbooks and MBPs should have no problem running on Apple designed ARM chips. The first to transition will likely be the Macbook Air, soon to be called the iBook ? Maybe iOSBook ?
If you look at their recent use of T1 and T2 co-processors in the MBP and iMac Pro you can get an idea of what may happen. I strongly believe they will start the transition by having an Intel CPU and an ARM chip both installed in iMac Pros and Mac Pros (maybe even a new Mac Mini?) that can share the tasks of running macOS programs and iOS apps at the same time. ARM chips are dominating the phone and tablet markets because they consume little power and run cooler. They completely outperform anything Intel does in that class of processors. It should be very easy to incorporate them into an iMac Pro or Mac Pro alongside an Intel CPU.
In this scenario, Mac desktop users can run things like FCP and Logic Pro and still have access to any apps that can run on a phone or tablet. This would make the transition much less painful for Mac users. The iOS developers should all be happy as their markets will increase and profits too. This makes sense and I hope it will happen, rather than a quick, clean break from Intel on the desktop and a complete reliance on just ARM based, Apple designed chips. We'll have to wait and see.
