Good point. Currently though, if you "z" a page you've closed, it preserves your input. The system isn't perfect, but it prevents any fear of accidental closing.
We think apps like Superhuman have brought the idea of single-key shortcuts into the forefront, and we really like how much more memorable they are for most users.
We'll definitely learn from our users' feedback though, and we'll work on a solution with them if they have issues with it :)