This actually works with GNU Screen. Because of this, and because of the lack of workspace reordering in tmux, I seem to be actually about the only person who likes Screen more than tmux.
There really isn't anything 'legacy' with screen. I've been using it for 20 years daily. I really have no reason to switch to tmux because I really can't see anything screen doesn't already offer me.
Vertical splits were patched in and added a long time ago, screen is usually installed by default, so I know it is there on systems where I'm not allowed to install packages. I can share screen sessions with co-workers, split windows etc.
What do you mean? Commands like tmux swap-window and tmux move window allow you do move the order of the windows / workspaces.