Because what you mentioned is ahistorical and based off reactionary history by bad actors.

Rms draws the line a GNU because GNU stands for a free operating system, which is what the GNU project is aiming towards. If this were purely a discussion about technicality, then we would be wise to let the matter drop, but that’s not whats at stake here.

shouldn’t I call my system Plasma/KWin/pacman/systemd/GNU/Linux?

You can, you literally can and it would be better that way to accurately describe what operating system you’re running. The shortest possible name is GNU, but that would be unfair to the contribution made by the linux foundation and the fact that multiple kernel projects do exist: so the name is GNU/Linux.

His essays on the topic which are publicaly accessible from the GNU website do discuss this.

other than to stroke his own ego

Rest assured that rms does not doing this out of ego tripping. Maybe you should tease Linus Torvalds for calling his kernel linux and the ENTIRE operating system linux. Torvalds is a multimillionaire who has used an apple M1 laptop. Stallman has never budged on libre software and directs his own life by his own stated principles. Call Rms stubborn, but never call him egotistical.

None of this is directed at you btw, it’s just something that always springs to mind for me whenever this topic comes up.

Please read Free Software, Free Society by Richard M Stallman so that this doesn’t have to keep springing up anymore. There are very few “linux” comm members who have read the foundational literature in full so I hope you do take my advice.