It’s not really the power of the processor that’s the problem. Instead, it’s the security protocols it supports.
Among other things, Microsoft wants to stop supporting the 16-bit bios that is used in aging legacy systems. So, this new OS is only going to support UEFI. That seems to be a done deal and that alone is going to eliminate quite a few processors/chipsets that probably have more than enough computing power to run the OS, which isn’t going to be architecturally dissimilar to WIN10.
At some point, they also want to make TPM the standard. But, if they do that, there are going to be a TREMENDOUS number of PCs that can’t be supported including devices that Microsoft sold themselves as recently as a couple years ago. So, that likely won’t be a final system requirement.
I get Microsoft’s concern. They’re starting to slowly lose market share to Apple and one of the reasons that drives people to the Apple ecosystem is security. Now that Apple has migrated to their own silicon, their security has only become stronger whereas WIN10 is a mess. UEFI and TPM would help Microsoft in this regard...minimally.
Agreed. Then there's the new instruction sets within the new chips that aren't within the old chips. Imagine a robot working in a tire shop. Version 1.0 of the robot might have to be told by the programmer every detail on how to take a tire off (i.e. step 1: make sure air tools compressor is on, step 2: put air hammer in right hand, step 3: place air tool against topmost lugnut, etc.). Version 2.0 might have much of that detail built in so that the only things the programmer has to do is say: Step 1-- take the lugnuts off, step 2: remove wheel, etc.). If 90% of the tire shops have version 2.0 or higher of the robot it might not be worth your while to make new software backwards compatible to using version 1.0 as well. It's the same way when making program code (including code in the OS) backwards compatible.
Finally, there's bloat. We demand a lot more from our OS than we used to. I'm old enough to remember when no one expected an OS to have its own networking protocols, much less a web browser. The idea was an IT dept would install the networking protocols specific to their system. Those days are long gone. Now we want an OS to do the networking, the automatic OS updates, manage device drivers and keep them updated, (and in Windows case) sync with Outlook email and Teams, etc.