Galaxies have in the past and still DO collide and there are also large objects in our solar system that can still do us harm even though they are not currently in line to hit us. Gravity changes all the rules of nice and neat orbits and anything passing by can knock them out of their current and presumably safe orbits. Even a passing star can stir up the oort cloud and send a planetary size comet our way...it's not a matter of if, but WHEN!