The underlying reasoning is that any system that is too uniform is fragile. In this case, the common belief that S&P500 is safe is valid up to a point, if too

The underlying reasoning is that any system that is too uniform is fragile. In this case, the common belief that S&P500 is safe is valid up to a point, if too many people rely on this assumption this can turn into a big problem. For most humans 99% == 100% but this is not true.