Holes in the Swiss cheese
Collisions in cruise happen now & then; Grand Canyon, Yugoslavia, India and Brazil come to mind. The first three were with older-generation nav and autopilot, which placed the flight path error on the order of a good fraction of a mile. Thus even if two aircraft were routed on a collision course, there was still a good chance of a miss - maybe with neither aircraft observing the other. The cheese holes are unlikely to line up, but occasionally they did.
The Brazil GOL/Legacy accident was quite different: The ATC error, coupled with RNP on two new/nearly-new aircraft, and neither aircraft using SLOP, meant the hole in the last slice of Swiss cheese was virtually certain to line up w/the other slices.
Which scenario will predominate henceforth? The proof is left to the student.