The unix philosophy is about what runs as processes at the system level.
I don’t know what you mean by “system level” (cat is userspace) but I don’t believe there is any clarification about what kind of applications should apply to the unix philosophy or not. It doesn’t say that applications “should do one thing and do it well only if it is a system process or terminal based program built for purely shell environments.”
Also, if the argument was exclusively about OS processes, dbus should be in the firing line of everyone in the anti-systemd camp too. That never gets the same level of hate.
The unix philosophy is old and, while nice to have, is insufficient to fully address the needs of the modern world. It’s not as simple today as it was in the 1960s and 70s and we need to embrace change to progress.
I’m honestly not sure what you’re talking about here. The unix philosophy is something informal that applies to compiled utilities that run usually in a bin/ directory. The philosophy isn’t attempting to apply to all software that exists and certainly doesn’t intend to apply to browsers or browser extensions.
