That’s fairly easily fixed, certainly if you’re Apple. They already have the equivalent x86 code (Swift and modern Objective-C couldn't exist without it), so they could detect the specific inline ARM code fragment and replace it by the equivalent inline x86 code.

At best, Apple announces mandatory bitcode* for macOS App Store apps at WWDC in 2019; Apple announces the first ARM Macs in 2020 or 2021. So you've got a window of maybe a year or two of apps that don't have to be recompiled to support ARM, and most are likely still being updated since it hasn't been that long since they recompiled to supply Apple with a bitcode version.

This doesn't do anything to solve the big headache of an architecture transition, which are those legacy apps that will never be updated again-- you'd still need an emulation solution like Rosetta or Microsoft's x86 on ARM stuff to support those applications. At best, you're maybe working around the Adobes and Microsofts of the world that move too slowly to have a day-1 update when the new architecture comes out?

* I'll note that Bitcode wasn't mandatory on iOS when it was announced in 2015, and it's still not mandatory there today-- it seems unlikely that Mac Bitcode would be mandatory immediately when they announce it or in the foreseeable future.

I’m not sure it’s really worth it today but this support would give Apple air cover for a future intel-arm transition.

It's really not too difficult to write a lifter, but it's time consuming. You have to take every instruction from the source instruction set and model its side effects in some IR. To make things simple, "whole binary" lifting tools seem to use a global CPU state (registers, flags, etc...) and so the IR you get is quite different from what you would obtain from a compiled program.