There’s one tiny little problem with the hijack scenario – people only ever paste from the primary selection immediately after selecting something themselves (and thus overwriting whatever was in there). That’s precisely because the primary selection is so easy to overwrite (basically when you select anything), so you want to select and then immediately paste so you don’t lose it.
So in reality this scenario in which a random page injects something in the primary selection and the user pastes it sometime later in a terminal, of all things, probably has like a million to one chance of occuring.
PS: Also, just for trivia, any user who’s been around the console for a while will hit Ctrl+C out of reflex when confronted with unexpected output, not Enter.