One thing that often gets overlooked in the tmux vs. alternatives debate is that tmux's staying power is largely about ubiquity on remote servers. Zellij is great on your local machine where you control the environment, but if you're doing a lot of work via ssh on machines you don't own, tmux is almost certainly already installed. The muscle memory transfers too.
That said, I'd push back on the idea that the default tmux config is just "ugly". The real usability issue is that the keybindings are so divorced from how people intuitively think about splits and windows that even experienced users can't remember them. The visual defaults are just a symptom of the deeper problem that it was designed for someone who already had a mental model built around screen.