In KDE's case the "you can just customize it" works both ways though. They could change the defaults and instead and let old users customize it back to the old

In KDE's case the "you can just customize it" works both ways though. They could change the defaults and instead and let old users customize it back to the old way it was, rather than every new user customizing it. It comes across as a pretty weak argument. The reality is that most users are simply bitterly opposed to change, especially in "subjective" parts of the system like UI design, and it has nothing to do with whether or not the change is actually an improvement that helps people, or can be undone with in 1 minute through a KWin tweak, or whatever. The very example you're theorizing about (accomodating new users who don't yet exist through UI improvements) actually has happened before with positive and negative examples e.g. Blender's complete UI overhaul in 2.8 which was widely praised, versus Gimp which continues to receive flack for its UI choices, versus Gnome which people just endlessly argue over both ways. It is not as simple as "New UI bad, old UI good" no matter how common of a mindset (and how over-represented) that is here. Developers of the project have to balance these concerns as they see fit, and that is their right. Being an older user of the project (or any user, actually) does not mean every decision and plan in the project is going to revolve around you exclusively, at the end of the day.