'Operating for 50 years without a collision'. Yes but the numbers of aircraft using Heathrow has increased significantly in that time. One collision in 20 years is merely a mathematical probability. It doesn't have to actually happen.
Not having happened in the last 50 years is good but it doesn't invalidate the statistical probability. I would say it hasn't happened because of the professionalism of controllers. Management and government have relied on that for too long.