I'll copy over what I wrote on Engadget...
I have my doubts on this 'rumor'... This would be an expensive transition, and I'm not sure apple stands to gain enough to do this (certainly not financially). Though, if anyone were to try (and succeed), it would be Apple. Of all the ARM companies around, they are the most equipped to estimate the trade offs to pursue this.
It's not that ARM ISA can't scale to desktop, it's just no one has bothered to do it yet. One of the biggest reasons why is you would need to sell a stupid number of chips a year to make this profitable. The closest I can think of is AMD, a fabless CPU company (lets ignore the GPU part). Even with their volume, they were still fiscally negative for years! This isn't something most companies can blindly invest in. It would require a team of thousands to make a desktop class processor. The time to market is pretty long so they would need to have started in 2017 (honestly, it should be pretty close to completion by now) to have something ready by 2020. Then they have to keep that pipeline fed to have yearly refreshes.
Emulation can be done (see Windows on ARM with x86 emulation), However, initial reviews shows it comes with an insane performance impact. At this point, you're better off with an iPad Pro running native code than emulation.
Binary backwards compatibility would initially be an issue, so I looked up the top apps on OSX (and the top professional apps on OSX), and most (all?) companies look like they're doing well so at least there's someone around to port the most popular ones to the new ISA. Most of the companies are small (few products) and a few are corporate giants. For the former, they must adapt or die, for the latter, they'll just do it because they have to. Given how (relatively) new the codebase is for x86 OSX, I'm willing to bet most of the source code is still around and (given good software practices... ahem...) easily portable.
Not to mention, if apple does this, all first party apps/suites/etc. would be ported from the get go. If your app is important enough, it wouldn't surprise me if you got an emulator/dev unit to port your applications before the official release date. If you're a big company like Adobe, I'm almost certain you will something before everyone else.
My bet is, if this actually happens, average user probably won't even notice that the ISA changed. I think this ISA transition will go smoother than the PowerPC to x86 one.
Hackintoshs/BootCamp/Etc. would all be in doubt...