Basically any OS can install any compliant terminal emulator, meaning the cases in which you really have to patch your TUI for another OS are few and infinitely more manageable than GUIs

Compliant with what? The proposal is "throw out all the compatibility and build some other kind of terminal" in which case nothing is portable.

How so? The idea is you build all the OS specific stuff into the terminal, not into the programs running INSIDE the terminal. Think Infocom Z-Machine, at the ultimate level, although obviously emulating a full on virtual CPU is overkill.

In any case we can surely do better than emulating DEC hardware from 50 years ago.

It depends. If the OS can run e. g. ncurses, you have access to quite a lot of software here, such as htop. Though, I am not sure if it works on windows; but I think it works on OSX. Windows is very strange.