There's no need to assume that a switch for some Macs will be a switch for all Macs. Multi-architecture binaries were standard for several years during the x86 transition. As they move to an App-Store model, it will be relatively simple to download the right binary based on architecture. Signed binaries could be required to be multiplatform. The minority of users who want to install software that's not Apple-approved will have to deal with potentially higher amounts of complexity.