KDE Plasma is NOT a Desktop Environment
What happened at the very early start of Plasma - if I understood it correctly, I wasn’t there - was the idea that Plasma had to be flexible to adapt to different form factors and devices. Even before Plasma Mobile there was a project called Plasma Active, with a very similar goal. So everything in Plasma was thought not as a Desktop Enviroment, but as a way of doing shells for different devices.
Or, as Aleix Pol said it during his above-mentioned Akademy talk, “Plasma is a UX for a number of devices, designed by us”.