And what developers will spend time and money to migrate their iOS apps to the Mac which has such a tiny user base?
I think you're looking at that the wrong way around:
currently a developer who wants to support both iOS and MacOS has to write and maintain
two different apps - many do, but they must be continually looking at the relative market sizes and wondering if the Mac version is worth the effort. Being able to target both with a single app would make them more likely to support (or continue supporting) the Mac.
Also remember that the iPhone market may be huge c.f. the Mac, but
last quarter the iPad sold about 10 million vs. about 5 million Macs, and I suspect that the 'iPad Pro' sales - the users who would be more likely to buy serious software - is probably only a fraction of that. So if you're thinking of writing some new software for the iPad Pro, being able to sell the same app to Mac users could significantly boost your potential users...
As for ARM-based Macs - Apple have carried off 3 processor switches: PPC to x86, 68k to PPC and 6502 to 68k (...but a 68k software-emulating a 6502 was faster than a real 6502 so the last one is stretching a point) - and in those days, far more software relied on lovingly hand-crafted assembler and direct hardware access than it does today. Many Apps written in ObjC or Swift that use the Apple frameworks should just re-compile.
Bear in mind that we
know that the next MacOS will drop 32-bit support, which is going to trigger a massive cull of "legacy" apps which aren't still being actively supported or contain too much architecture-specific code to be economical to fix.
The big problem with ARM Macs is - as people have noted - bye-bye BootCamp/Intel VMs. Sure, the ARM can do dual-boot or virtualisation, but it will be virtualising ARMs (and, apart from the IP issues about x86/AMD64
emulation, the ARM doesn't really have the performance headroom over Intel to deliver very good performance). So its gonna be virtual Linux or BSD (which is fine if you're doing web development or want to run Docker or suchlike) but even if MS start selling ARM Windows retail its probably not going to scratch that legacy software itch very well. I'm not going to try and put a positive spin on that, or tell people they don't need Windows on Mac, but I suspect that its a sacrifice that Apple would be happy to make.
Now, if I ran Apple I'd... well, I'd sell up, buy an island somewhere and enjoy a luxurious and idle retirement but that's not important right now

... er... if I were to armchair quarterback, I'd say
don't start with an ARM Mac/MacBook, instead, put MacOS for ARM on the iPad Pro (and make a keyboard case with a trackpad). That way, you're adding features to the iPad rather than removing Intel compatibility from the Mac. Five years down the line, it may all be the same. Hardware wise, the latest iPad pro is more interesting than any current Mac laptop, and even if the notion that its as fast as a MBP is... optimistic... it's clearly not chopped liver.
The way Apple could easily get this wrong is to pull the rug too soon - they'll need Intel and ARM Macs running side-by-side for a couple of years, and that means
not letting the Intel Macs languish with out-of-date processors. The idea that they might use it as an excuse to lock down MacOS as much as iOS is also depressing - although we can probably kiss goodbye to Hackintoshes a year or two after as Apple have finished rolling out the T2, anyway.
I can't be
too pessimistic about ARM desktops, since I was using one in 1988, and back then, the original ARM 2/3 ran rings around anything Intel had to offer. Acorn/ARM stopped designing desktop chips concentrated on embedded, and then mobile, applications because it was the only way they could sell processors that didn't run Windows.