We are talking about CSD, are we not?
There are quite a number of DEs using CSD, and a couple of them even feature a more "traditional" layout with a button panel and left side app menu.
While some folks like it, at this point one of the defining features of Plasma is the fact that it stands out by not using Server Side Decorations, or GTK for that matter.
~~Besides, CSD is ugly~~
Not really following what you mean.
The moment you allow CSD, apps can and will put whatever they want on there, leading to wildly inconsistency in the number of things there.
CSD will always looks weird cause it takes too much vertical space. It will never look as good as a normal Title Bar, which can be controlled by the server.
You also lose draggable space, as now buttons are taking space.