I use Sway and happily manage my terminal windows using its tabs and splits. I can mix other "graphical" apps with the terminals or move them to another screen, all with the same keybindings. I agree, though, that I wouldn't want to replace my Sidebery vertical tabs with this setup, but I ultimately don't see why equivalent functionality couldn't be implemented inside the window manager in some alternate universe.

There are sadly hardly any WMs now with tabs, so having them support tree style tabs as well doesn't look too realistic.