I'm an i3wm, neovim, zellij user and WoW player. I have so many bindings that I use to navigate or do things, and I've optimized/automated away things that are repetitive to me.
Zellij follows the unix philosophy in my book of being a good multiplexer. Sure, Intellij is also a sort of multiplexer with panes and terminal Windows, but I can't use spotify_player and navigate around it inside Intellij like I can within zellij.
Essentially I like installing a tool for doing one thing well, and then combining tools to make my environment fully usable. It allowed me to super easily swap out tmux for zellij when I found I preferred it, but I can't swap out Intellij's paning system for VScode's if I find I prefer it.
zellij is a terminal multiplexer like tmux which for example allows you to have multiple terminals shown in the same window. It works both locally and over SSH. Which GUI would do the same?
I'm currently using zellij after previously having used tmux. For me zellij is a direct upgrade in every way, and the default tmux keybindings are even supported out of the box in zellij which makes transitioning incredibly easy. The only downside is that I have no idea how to pronounce the name.
Wouldn't the multiplexer run inside the terminal emulator? I don't use a multiplexer myself, but I thought it was independent of what emulator you use.
GUI just doesn't give you the level of control or keyboard driven workflow that CLIs can. You can even use plain kernel terminals or bootloaders if you know CLI. Text config files are also extremely easy to configure. There are only a few legitimate cases where CLI isn't enough. That includes gaming, 2D and 3D graphics design and CAD/CAM. And almost all of them implement their own GUI anyway.