This is a tough one for me...
I know that the ESC key isn't going away for us Windows users, but I'm trying to take a birds-eye view of the whole non-console gaming ecosystem.
If I had to shell out $2400 for a Mac without an ESC key, I would not be very satisfied with using a touch screen to "cancel" menus. I need that tactile sensation. I would personally bind the ESC key to the Tilde key. But for GUI design for the general I-don't-want-to-mess-with-settings Mac crowd, that is rough...
I don't know how I would handle it if I were a GUI designer. I never thought of how important that key would be until there was talk of removing it. I think of all the times I hit ESC to close a menu in games, and I cannot think of a good replacement that is tactile.