All things UI in Linux distributions go through so much insane thrashing that very very few application developers want to bother. There are no tangible benefits to UI thrashing; people use systems for software, not window chrome!
This is because the developer community, aside from some long-time system hackers like Linus who know what's at stake (most of whom are Linus's age or older), do not think in terms of building software that will last and adapt to changes. They think in terms of "old and busted" and "new hotness" and that their job is to migrate everyone off the old and busted and force the community to adopt new hotness. Hence the churn in GNOME, hence systemd.
I'm not up on the latest in Gnome-land and don't know the background on the issue you linked, but it looks like the benefit is that they get to stop maintaining a bunch of code for a UI feature they've decided isn't actually a good idea. That will make it easier for them to continue to build reliable software, as there will be less legacy code to consider when writing future code.
The UI churn you talk about may be because the "window manager" concept is probably fundamentally flawed. Creating a coherent and sensible UI when you have to target a whole suite of different window managers which may use entirely different UI paradigms is... probably a nigh-unsolvable problem which might not even be worth working on.
The only time you run into a WM issue is when you try to ape the OSX singular menubar.
As long as you go with the Windows thinking of having the menubar inside the relevant window, your UI will work across just about every WM out there.
I really don't see why the UI world is so hell bent on moving away from the menubar. That is unless they are stuck up on turning every UI into a touch UI...
This:
https://trac.transmissionbt.com/ticket/3685
All things UI in Linux distributions go through so much insane thrashing that very very few application developers want to bother. There are no tangible benefits to UI thrashing; people use systems for software, not window chrome!