Overall I don't think it is off topic.
If the system is in
/System, why isn't an OS upgrade simply creating a
/NewSystem, putting all the necessary files in it (all in the
background, without interrupting the user), renaming
/System, and then rebooting?
Is basically asking why doesn't Apple just create
/NewSystem... So I can see how it may be perceived as
Asking how/why Apple does some thing XYZ.
when in actuality (I think) you were asking more like "wouldn't it be faster to do x"?
Edit for clarification
“Wouldn’t it be faster to do x” is still borderline off topic. I think you were getting a bit confused because what you intended to say and what you did say were different. And people marked what you did say as off topic.
Even without that paragraph it would basically be why is updating so slow. Which could be marked as too broad.