But why do they need another DE when Gnome and KDE can suffice botrh demographics and are vastly more mature and see more dev effort?
Clearly the people developing it aren't happy enough with Gnome or KDE and want something different. What matters is not total amount of dev effort, but if what comes out of that is something you like or not.
Aside from the technical issue mentioned in links, the ability to execute efficiently on a long-term vision needs autonomy which isn't compatible with working within a large established development group with their own directions. I personally still choose Xfce, but welcome to seeing what System76 comes up with and may switch down the road if it strikes a good balance between simplicity and capability available to power users.