There's no technical reason why Mac OS on ARM should look, feel or work any different to Mac OS on x86. Its 2020 and if an end-user needs to know what CPU their machine is using then someone is holding it wrong.
Any change, any further iPadOS-esque lock down or restriction is purely a marketing decision for Apple.
Apple seem to have a successful and separate product line with the iPad Pro targetted at the "2 in 1"/MS Surface market, so it doesn't make much sense to turn the Mac into, basically, a big iPad with a keyboard when they already make a big iPad with a keyboard.
...on the other hand, if Apple
did want to turn the Mac into a big iPad with a keyboard, they could do that tomorrow without switching to ARM.
Not to say it couldn't also be very different - OS X was made from the ground up for the Intel transition
OS X was made from the ground up (starting with NeXTStep in 1989 initially running on a 68030, later SPARC, PA-RISC and Intel) as an implementation of not only Unix - an OS founded on source-code level compatibility that is pathologically hardware independent. OS X also allowed Apple to easily spin off a mobile-optimised version for ARM (iOS). Anyway, OS X development started with the purchase of NeXT in 1996 and, at the time, had to be ported to PPC, so while the potential for moving Macs to x86 (or, at the time, SPARC, Alpha, or even ARM) may well have been a selling point, the actual Mac Intel transition was still a long way off... The priority then was that Copeland - the planned successor to MacOS - was an epic fail and Classic MacOS was long past its sell-by date.
Without any third-party apps. And without Bootcamp.
Well, Bootcamp as we know it would be over - although Bootcamp for ARM Windows would be technically possible. Third party apps, though... huge swathes of them will just need the developers to tick the 'ARM' box in XCode, re-compile and test, many of the likely problems will have already been fixed by the 64 bit transition, while tearing out any x86-specific optimisation and replacing it with calls to OS X frameworks is progress, since it means they will be able to use future hardware acceleration features that Apple may include on their custom processors.
Apple have done this twice before at times when it was
far more likely to run across lovingly hand-crafted assembly language or hard-coded AltiVec/MMX instructions in source code. They've spent the last several years promoting hardware-independent code via their app store guidelines which will help.
Some of the big pro Apps with their huge ecosystems of fourth-party plugins may take a while to get ported - but if Apple have any sense they'll get that ball rolling months before they actually release ARM-based macs, let alone start discontinuing the Intel versions of higher-end Macs (I don't see them dropping the Mac Pro next year unless its a flop).