At that point what you want is just a tiling window manager rather than a terminal that supports GUIs.
I already do use tiling window managers and they don't really accomplish the “if you launch a graphical app in a terminal window it takes over that terminal window” flow. Closest I've found is Niri's support for tabbed windows, but even that's just sticking the graphical app window on top of the terminal window instead of the terminal window itself becoming the app window.
Depends, I'm building a markdown down editor that just previews in the web ui. However, I can use the web ui to do tasks like uploading files, view git commits etc. Different interfaces for different purposes. The CLI gives me focused mode, and then visual stuff that steal my time goes in the web