I agree with ARM chips starting as coprocessors first. The same way the T2 chip handles certain very specific tasks, i think that's how ARM will come to Macs first. ARM chips might replace the T2 chip or might work in tandem with the Intel CPUs to handle low power/MacOS specific tasks and discretely (or simultaneously) switch and use the Intel CPU for third party applications, similarly to how Iris and the AMD graphics cards tag in and out to handle the different levels of graphics tasks. I could see that being the method they do for a couple years to give third party developers time to start coding their apps for ARM and then start giving more and more responsibilities to the ARM chip until eventually Macs with ARM chips as the sole CPU are ready and can run everything normally the way x86 chips do now.