There's no reason why it shouldn't. You seem to think that the interface obliges a program into a certain performance pattern. No such obligation exists. And Emacs isn't a TUI program, it only happens to have a terminal interface among many others.
I think all software (or at least, any text editor) regardless of interface type should launch instantly. But it's more unjustifiable with TUI programs.
No it wouldn't. Those interfaces are permanent and only get restarted once a day or if the hardware has to be rebooted. Same for Emacs: there's absolutely no need to start the editor every single time.
> You haven't made a case for why some user interfaces shouldn't start instantly
I'm not making any case, we're not in court. Startup time is irrelevant and your fixation with it is really funny (up to a point).