In response to ew0k and Toby
While I'm not a programmer I love the terminal and have thought about the terminal as an extremely lightweight (and possibly cross-platform) interface before a number of times. I do use my terminal as an interface for my dashboard including buttons for mouse/touchscreen interaction. To answer ew0k regarding mouse clicking in the terminal, yes most terminals support mouse interaction. Not all but most. My terminal dashboard has a keyboard attached only but runs on a Pinetab with a touchscreen. I switch tmux windows and panes, tap buttons for controlling music etc. via the touch screen in the terminal. The terminal emulator sees the taps on the touchscreen the same as a mouse click.
Regarding Toby's comments about a smartphone using a TUI as the interface. SXMO, which is what I run on my Pinephone, is partially doing this. It's not full TUI through and through. The interface is based on sway wm but most of the built in functionality is shell scripts and terminal apps. Some basic navigation in terminal apps can be done via touchscreen gestures. The performance benefits are pretty clear especially on a low-spec device like the Pinephone. SXMO is noticeably snappier than any of the other Pinephone OSs I've tried.
There are probably tons of options for making TUIs, ncurses, python, this TermUI go framework that I have bookmarked. I think we need more TUIs :). They are super lightweight and I think they look cool.