You can get a long way with neovim. I use it as my text editor, ide, and terminal multiplexer. Consistent tabs, panes, text editing, etc. If it supported a terminal graphics protocol it would be just about perfect.
How do you switch to other apps, or between virtual worksapces though? All that's defined by the terminal emulator, or window system, outside of nvim, and hopefully it's consistent or at least doesn't conflict.
So I guess the OP argument is that everything should be in the 'window manager' garden, instead of all the apps making their own little alcoves with their own custom little languages.