My opinion is default install should be to CLI or at most good old twm.
I don’t want a default install that is huge and bloated; that is what you get with any KDE/GNOME install. LXDE, XFCE are lighter weight but still more that I would desire for a default. Lumina is a reasonable alternative; right now the only dependencies are QT and FluxBox/OpenBox. Lumina 2.0 whenever it gets here should be only QT.
Everyone has an opinion on the “best” Desktop Environment, I think some people actually like Windows 8. Me, I don’t need or want an Environment, I want a Window Manager. I’ll take care of the rest.
KDE is not strictly Linux, but over time has pulled in more and more Linux “features” like dbus and other things. That makes the BSD ports difficult: do we pull in dbus et al or build without? Build without, what is lost?
Every “supported” DE is part of the testing matrix that if one has unlimited resources, sure, could be done. But what happens if you get someone that likes the KDE file manager, the Gnome webbrowser, the Lumina Calendar and they report problems. Who is the blame? That’s why there Ubuntu, Xubuntu, Kubuntu, Lubuntu and whatever else Ubuntu.
Circles back to default install should be CLI, then maybe on first boot you pick what DE you want, it gets downloaded and installed at that point.
Or default install is Lumina, user installs something else if they want.