I think it's worth separating the two related but distinct concepts of what is a part of the operating system itself (for example, the actual file manager) and what is pre-installed or bundled with the operating system (games like Minesweeper).

I agree with you that a rich text editor definitely shouldn't be part of the OS. But should it be a bundled part that ships with the desktop environment, the way Windows/MacOS/Android/iOS/ChromeOS all come with photo library software, basic image editors, media players, browser, email client, etc.? These applications aren't strictly necessary to use or maintain the system itself, so maybe they shouldn't have some kind of privileged use of the OS's functionality, but there's no harm in bundling in the installation defaults.

I don't think a rich text editor is an important enough function to necessarily be preinstalled with the OS, but I can see an argument, at least. There's a reason why Windows shipped with one since the beginning, and why MacOS and KDE and Gnome each have a default that very few people actually use regularly.