I find these rumors continually terrifying. Boot Camp and VMWare Fusion is vital to my workflow. If Apple drops x64, they're dropping me as a customer.
I'm sure if apple switched entirely to ARM, lots of other software would switch too, and we might even eventually see the end of the x86 era.
Windows is just one of many pieces of software people would need to run on an ARM-based Mac. Seems pretty clear that this can only work if the ARM-based machines have a pretty decent way of executing x64 binaries.
Windows is also pushing more ARM support. As a result, some of the infrastructure e.g. Chromium and Electron is starting to get ARM support. I expect other tools, platforms, and frameworks to start getting ARM support, but it won't be quick, or uniform. This will speed up efforts across the industry. I think they really need to break with the past to get the huge battery life gains that are possible, and Intel/AMD don't have much to offer there.
I don't think any of this will happen overnight, but Apple will look to move things quickly. I'm not sure it's the same situation that Apple found themselves in when they moved off PowerPC. That was an existential issue for their hardware. Now they are on the incumbent platform, so there is time to make a more considered transition. They have iPad as a place to experiment as long as they maintain the general usability and experience of that platform.