The group Kwin developers have the most to fear isn't other KDE developers, KDE apps do and always will play well with Kwin. It isn't even individual apps that change everything like Chrome. What they, and every KDE user, has the most to fear is the numerous GTK apps that up to this point have been able to work reasonably well with Kwin and Plasma features, but with CSD decorations would suddenly break KDE users' long-established workflow.
This also explains why Gnome has been okay with CSD. Qt (and thus KDE) apps have supported GTK theming and HIGs for a long time now. So Gnome doesn't have to worry about KDE apps suddenly breaking established workflows, they know KDE devs will bend over backwards to maintain consistency as they always have.