You can't just pass through things; what happens when you resize, (re)attach, switch tabs &c.
Tmux needs to know that there are graphics on the screen and redraw in any of those situations.
Terminals and shells aren't like window managers where you can just subdivide your drawing space and pass a block pixels to something else to draw on.
What the terminal processes processing is a stream of bytes from the shell and needs to be able to interpret them in order to lay things out correctly.
Is this as big as bring a stacking window manager to tmux (compared to the current which is tiling)? Or is just better graphics in the terminal? or am I way off here?
If it's just the better graphics, then I'm not sure I understand why tmux needs to support it. I would think the terminal emulator (such as Gnome terminal) is what would matter. I suppose maybe both but I would think tmux would just pass through.