I've spend a reasonable part of my career with pieces of my own airplane a few feet or inches from other airplanes in flight, sometimes going the same direction, sometimes going the opposite, and can only say that assumptions will kill you.
I've lost friends and associates who met other airplanes that occupied their piece of sky, and such assumptions killed them, too.
The "big sky" theory was always flawed.
Don't forget that the first two automobiles on the face of the planet were involved in the first auto collision...and certainly airplanes continue to collide, too. TCAS doesn't prevent it. Controlled airspace doesn't prevent it. Where do most occur? In VFR conditions in controlled airspace while talking to ATC, in radar contact. Go figure.
When a bystander sees something and has a question, simply throwing out the assumption and big sky theory really does little to answer the question, or allay the concern.