Tmux undoes all the fancy performance end ergonomic stuff they spent their time and energy developing. Moving stuff to the GPU, multithreading, etc. -- all just to call tmux anyway and lose 100% of those features!

What we really need is a protocol for tmux (or other terminal multiplexers) to expose its internal state to native terminal emulators so that you can get the best of both worlds. That's a tremendous amount of work, though.