Slightly off topic, but I sincerely hope they do not do this. For a decade now MacBook Pros have been ideal developer machines. Mac, Windows, Linux, UNIX, mobile Android, and mobile iOS (granted the last two are emulated; but the dev tools are x86). Just launch a VM for the other OSes and you're good to go. Any office I visit, any meeting with any client, any project I'm doing...everything is on my MBP.
I've run into developers who are not primarily interested in macOS or iOS yet use MBPs for their work.
A switch to ARM would kill the ability to host multiple OSes on one notebook. The ideal of having all your projects, files, and test environments on one machine would be dead. I hate the thought of having to lug around multiple machines. And quite frankly macOS is the best VM host for dev purposes. I don't want my primary/base OS to be Windows or Linux, nor do I want to try and VM a semi-legal copy of macOS on a Windows or Linux notebook.
I remember the RISC v. CISC wars of the 90s. Backwards and cross compatibility has trumped every RISC advantage. The only reason ARM succeeded in mobile is because there was nothing to be backwards or cross compatible with, so efficiency won. But x86 has literally 41 years of legacy desktop and server code. There's zero reason to walk away from this.
Now if they want to start exploring AMD processors on the desktop...I'm fine with that :-)