There’s just no way. With PPC to Intel, there was Rosetta which kept compatibility for PPC on all Intel Macs from 10.4.4 (first Intel version) all the way until 10.7. Then, the 64-bit transition was a loooong time in the making but they first warned developers in December 2017 by requiring 64-bit in the Mac App Store, and external apps were still supported with a warning until 10.15 (released October 2019).
Switching to ARM would most certainly be completely incompatible. Unless they manage to pull off another feat like Rosetta (which would likely be MUCH harder in this case), it seems extremely unlikely that they would switch to ARM and break compatibility with all apps considering that they have yet to warn developers.
I think there’s a good chance, actually more likely than not that they will eventually switch to ARM for many reasons (efficiency, unified architecture across all Apple products, etc.) and some non-Apple portable PCs already use ARM. However I think that’s a long ways away, unless they run iPadOS. macOS isn’t ready for ARM.
Following your train of thought, OS X was not ready for the transition to Intel CPUs either back in the day. Yet, they magically pulled Rosetta out of thin air and the thing took off, starting with a a brand new iMac and a new series of notebooks.
The strategy was simple and effective: Start with a few models for the mass market, then transition the high-end product line later. In other words: Give the one and only relevant third party software supplier in the Macintosh market -- read: Adobe -- time to migrate.
There are three options how they can transition this time:
Either
-- Release another Rosetta-like CPU emulator, only this time for Intel CPUs instead of PowerPC processors.
Or
-- Put an Intel CPU in the mainboard as some kind of co-processor that will still natively run x86 code, but run macOS (or some kind of iOS that it will probably rather be) on the ARM processor.
Or
-- Offer people who buy ARM-based Macs some kind of remote desktop solution: The old Intel software will run in Apple's cloud and you access it via a remote desktop client. This is what Microsoft is already offering under the name "Windows Virtual Desktop" for corporate customers as an Azure service. But, let's face it: Apple neither has the technology nor the data center infrastructure for this approach (unless they become a Microsoft or Amazon customer).
But: If they want this transition to happen, there are ways for them to get there. They've done it multiple times before, and each time without an early warning. The Intel transition was also only a rumor until it was announced on the same day that they started selling the first Intel Macs.
For Apple, it makes strategic sense to move away from Intel to an own hardware architecture (again). However, this time around, it will become a big problem for many customers: Boot Camp and the ability to natively run Windows on Apple hardware will be gone. Virtualization solutions to run Windows software on macOS (in a VM) will become painful again and they will have a significant performance loss compared to current solutions offered by VMware and Parallels. People who need to run Windows or other operating systems side by side with macOS will need to look for better solutions.
