Maybe the switching costs are high, or maybe not. Superhuman is designed such that every single interaction can be done on the keyboard, I'm not sure of another email client that has that level of control, so I'm not sure as to your point about memorizing keyboard shortcuts.

It's like an email version of vim (or emacs if you prefer), but more GUI-based and for non-technical people. Would you pay for vim, is I think the real question, given that one is an active vim user already. Since it saves me so much time and switching cost from keyboard to mouse, I would pay for it. That it is currently free and open source is nice, too, but I think some programmers would definitely pay if it weren't.