Screen is for the most part "good enough" that any replacement that aims to supplant it completely needs to either be vastly and obviously superior or enough of a drop in replacement that it doesn't require any effort to switch (I'm most definitively not investing time updating config files or changing key presses unless the improvements are dramatic).
As far as I can see, while tmux might be "better" in some nebulous way, the value proposition is way to weak to invest time in switching for me.