Why do you say this? There is nothing fundamental about text. Is there something fundamental about text in Smalltalk? Or AmigaOS?
> There's just no standard or integration, and tools that allow that (like a system-wide middleware)
COM on Windows. Scripting interface to apps on MacOS. They're there.
This is a reality within Emacs (which really is a 2D text OS running lots of applications inside, including a text editor), and being text-based does play a role. When it's very hard to draw arbitrary pixels on screen and most of all apps deal with text, it's easy to make a large set of very powerful interface tools, and it's easy to pull data out of an app and put data into it, whether the app intended it to happen or not.
In the back of my mind, I sometimes wonder how something like Emacs could be made with modern browser canvas, to enable cheap rich multimedia, while retaining the ability for inspection and user-programmability. Introducing arbitrary GUIs is hard, because next thing you know, half of the stuff is drawing to canvas directly and it's all sandboxed away from you.
I think this is why it reminded me of TCL, specifically the "expect" command that can script apps that know nothing about it. From the Wikipedia page, the TCL Expect extension: "automates interactions with programs that expose a text terminal interface".
So how I imagine this "Emacs as an OS" paradigm you're describing, is that it mediates interactions with any and all apps that expose a text input/edit interface, to allow programmatic customizations.
Like I'd love to script my own shortcuts for Firefox (or other apps) - possibly with multiple steps, taking input from some config file, or sending a link to another app.. Or, as you mentioned, Emmet-style expansions that work in any input field or textarea..