That is false dilemma between 'only compositor can handle screensavers / global keybindings / screen grabs' and 'arbitrary app can grab screen / read all keystr

That is false dilemma between 'only compositor can handle screensavers / global keybindings / screen grabs' and 'arbitrary app can grab screen / read all keystrokes'. It is easily conceivable to have a system, where regular applications cannot do these things, but privileged programs can do that (without being integrated into one big compositor).