Well, I do not know if I could totally agree with what you say, a year ago I had been doing it like this and for some reason not all things are “updated” in the same way if you do it from a new iso, what I mean is that, even if you do it by hand “sudo pacman -Syyu”, many things in the desktop environment that have changed are not reflected in the old installation.
Last week I maintained a laptop and a desktop PC in my home, as those teams were already more than a year old and also used the “testing branch”. When we decided to give them to another member of the family, I decided to install the latest iso instead of running the upgrade. A quick example of what I found was the totally different pamac GUI, even my personal PC totally up to date, it didn’t reflect many of those changes, neither was the terminal shell change there, nor was the home screen improvement using UEFI and displaying “manjaro” correctly at startup.
Also, and as a curious and positive fact, with the laptop, I did not have to look for the WiFI driver anymore, it was a realtek xxxde that now seems to be supported, but throughout the year it had given me problems after the updates to the point that many Sometimes I must have recompiled the driver after a kernel update.
Anyway, it’s just my experience with this, in our family all the PC’s, even a MAC Mini were migrated to Manjaro more than a year ago, none with dual boot or strange things.
So yes, the change was noticeable in all of them … at the end of the day, after seeing so much change in the other computers, I decided to do the same with my everyday PC.
Well, I did find changes, the vast majority very positive, it seems that after the great update of Gnome 3.38 and the recommended kernel for testing branch, somehow my PC was not assuming them with the upgrade.
Hope this serves the developers. I repeat, it happened on each of the 3 machines, so I don’t think it’s a coincidence.