@futzle similarly, back before GPS, radio altimeters, etc, if two flights were accidentally assigned to the same flight level, differences in their barometric altimeters meant they would probably miss each other even if they got quite close.

So after the tech advances, everything is right at the altitude it’s been put at and the danger of collision went up for a while.

That’s fixed now, but only because they built an entire traffic avoidance system to avoid this (TCAS)