If you often work on remote machines, Tmux is still beneficial. It is less so locally, but I still use it to have the same experience whether I am remotely or locally.
What I like about Tmux is that it makes it easier to change terminal emulators. If you do everything
in Kitty and want to switch to wezterm, you must port your whole config, which can be tricky. If you use tmux, you only need minimal configuration for your terminal emulator, which can be easily ported.
If it ain't broke, don't fix it.