I remember when I was playing FNaF World and made a lot of progress!! Then I accidentally clicked ESC...no emulator that i know of uses a single button to exit as default. i personally would want to do that. i play a lot on my keyboard and it is possible that i would press ESC accidentally and close the whole emulator, losing my saves. ESC for getting out of fullscreen is good. dolphin uses it to pause the game, and that is because it supports multiple screens and susupending emulation. pcsx2 as does the same.