What about the Manjaro users who do not use the GNOME/GTK-ish flavour (and want to use GTK2/3 applications as less as possible)?
Let me reverse your motivation for your proposal for you:
I think merging the Manjaro Settings in to with KDE’s System Settings application would make a more coherent user experience.
Do you see what I did there?
So are you asking a dev team to maintain a fork of the system setting application of each desktop environment, instead of just one application?
Or are you asking for a more opinionated direction of the Manjaro project which result in some users are becoming “second class citizens”?
Manjaro settings manager already integrates to xfce, mate and kde system menus. OP is just wanting the same user experience across the different desktop editions, which is quite reasonable thing to wish for.
Unfortunately it is not feasibleto implement. I spent a few days researching the matter and contacted gnome developers regarding them matter. Gnome settings center is not pluggable like the other settings centers are. If we wanted to plug msm into it, we would need to ship patched version of gnome settings center. This is not feasible with our resources. I would like to do it, but I simply don’t have enough time for it.
If some enterprising users take it up to themselves to write a suitable patch, I’ll be happy to package it. I probably wouldn’t plug msm in, but call directly mhwd in the background and make a custom interface following gnome hig.
You may ask: how can Ubuntu ship patched gnome settings with extra options and manjaro can’t? It is simple: manpower and money. Ubuntu has a bigger team, full time developers and focuses only on one desktop environment.