> What we've never seen is a unifying system that treated the TUI itself as the primary user experience.

Do things like https://en.wikipedia.org/wiki/DESQview from way-back-when not count? I've seen more modern systems trying to do similar though they rarely offer enough beyond tmux+byubo, tough state plans to, so don't get much traction and drop into being unmaintained before getting to the point of being stable.