...and I bet a lot of the effort is going into the 'reimagining' bit - the whole user interface needs to be re-designed to work well on a touch-only device, and although the iPad pro is impressively powerful for a tablet, its specs are quite limited c.f. what you'd expect from an ARM Mac. Photoshop for ARM Mac doesn't need a re-written UI.
Why? x86-on-ARM emulators have existed since the 1980s. Microsoft already has a "modern" x86-32 emulator for ARM as part of its ARM version of Windows. The only issues are legal ones which, unlike the laws of physics and mathematics, can go away if you throw money and lawyers at them. In any case, its only ever been a stop-gap, and the performance won't be special. However, 15 years on from the last switch, the software world has moved on and more applications should be in the ideal "tick 'ARM' and re-compile" class than ever before.
Do you mean the messy end of the PPC era - which was caused by Apple being totally dependent on Motorola and IBM producing the new processors it needed - like a mobile G5 for the PowerBook? Because the #1 point in Apple switching to ARM would be to lose their dependence on Intel, who are now causing them similar problems (not just Intel's current supply and die-shrink problems, but also their regular delays in releasing the particular power/cores/iGPU combo that Apple need).
There seems to be a bit of revisionist history going around suggesting that Apple switched to Intel to get Windows compatibility. They switched because development of G5 had dried up, whereas Intel had just dumped the whole Pentium-4 space heater dead end and started making decent processors (the Core series). It was hackers (in the good sense) who first showed how the x86 Macs could run Windows - the first x86 Macs didn't even include the EFI BIOS emulator module needed to run Windows, which the hackers had to find and install. Bootcamp and Parallels followed later.
Only if (a) Microsoft/Apple make ARM Windows available for Mac and (b) your applications work with the x86-32 emulator in ARM Windows.
By charging more. Remember, that the Xeon processors in the Mac Pro run from $1200 to $7500 "recommended customer price".
You may have missed the recent announcements of 64- and 80- core ARM chips from multiple sources. Not saying that those are drop-in options for the Mac Pro as they're designed for server workloads (although even the Intel Mac Pro depends on mooooar corrres!!! for its performance), but they show that you don't have to be Intel to develop high-performance, custom ARM chips.
Still, yeah, the Mac Pro is going to be the biggest challenge.
They need developers to start porting their programs, and an iPad Pro running MacOS might not quite cut it.
(Assuming by "platform" you mean "hardware platform" rather than "Operating system")
Linux, as with all Unix-like OSs, has always been focussed on source-code compatibility rather than binary compatibility, and the majority of the big open-source projects are already up and running on ARM Linux. Heck, I was using Unix on ARM (RiscIX) back in the early 90s - it took me an afternoon to get the then-standard HTTP server up and running, and that was due to a minor difference in Unix dialects rather than the CPU.
MS's current preferred development tools - whatever they're calling .net these days - compile to bytecode which runs in a virtual machine (Common Language Runtime). Of course, Windows has a "legacy" software problem beyond the dreams of Mac users - the 32 bit version still runs Windows 95 binaries.
Apple have spent the last several years persuading developers (via App Store rules) to use standard frameworks like Metal, Accelerate, Core-whatever etc. rather than use hardware-dependent code. Any new-ish Mac software written to Apple guidelines is likely to re-compile for ARM with little if any changes. The last big change - the move to 64-bit (which can affect data types and structures in source code and is a potentially bigger deal than switching CPU) - will have mopped up a lot of potential problems with ARM64.
Then you've got the huge increase in applications developed using web technology - HTML5/Javascript - which are genuinely truly platform independent (its not just websites/online apps - MS Visual Studio Code is mostly Javascript running on a bundled version of the Chromium browser).
What people just don't seem to get is that the vast majority of modern software is written in high-level-language, and only uses hardware-specific code where absolutely unavoidable . Modern operating systems have built-in hardware abstraction frameworks (e.g. Metal, Accelerate in Mac OS) designed to avoid that need. Even drivers can be mostly written in high-level language (as is most of the OS itself).
Supporting multiple operating systems is a major pain because it usually means a completely re-written UI and different frameworks (e.g. Metal vs. DirectX). Supporting the same/similar operating system on multiple CPU architectures is a breeze by comparison.
Once you've fixed any issues that stop your MacOS x86 application running on MacOS ARM there will usually be no need to maintain separate sets of code for x86 and ARM - you'll just build it as a universal binary (it might even be possible to use bytecode, in which case the App Store builds CPU-specific binaries). It won't be zero work - you'll need to do some testing (oh, wait, what am I saying? Its 2020 and everything is beta) on both platforms - but its nothing compared to supporting multiple OSs - even MacOS and iPadOS.
Yes, there will be exceptions - a small number of special cases where you have to use CPU specific code (maybe some game engines are still written in lovingly hand-coded assembler?) but far fewer than there were at the time of 68k-PPC or PPC-Intel - the last being a switch from big- to little-endian which can also affect high-level code). ARM is 64 bit, little endian (well, technically bi-endian) just like Intel. It will be using the same compiler front-end so there shouldn't be any C/Swift dialect quirks or different data sizes. Nothing like this is ever trivial but there's no reason for it to be much worse than the usual annual OS release or the 64 bit switch.
