Z is only depth if your camera happens to be at the origin facing in the positive Z direction, though. In most games, the camera almost never rotates except about a vertical axis, though, so Z as the vertical axis stays vertical always. (Exceptions being space sims, that leaning-around-the-corner maneuver in a lot of games where the camera tips, games with shifting gravity, etc.)

I dunno. Z as up always felt more intuitive to me. It's just another thing to argue about like Vim vs Emacs and tabs vs spaces, I guess.