I've been using screen for years. It works well for me. Can someone list some reasons to switch to tmux? Just about the only thing which annoys me in screen is the awkward switch to "copy mode" to scroll, instead of just hitting PgUp.
tmux isn't a dead project.
Spirit of exploration? I started with tmux and no screen experience, but I've converted a few people to tmux by them just giving it a chance, basically.