Systems fail in a final cascade after a certain amount of fault corrupts their operation. It can be a large fault or many smaller faults or any combination. If the system is self-correcting then time becomes the most important factor as the small faults can be corrected one at a time as they occur but usually will be overwhelmed by too much at once. The human race does not seem to be dealing with various problems in any logical fashion and it does not look good for us.
Amazon women on the Moon might save us but we have wasted way too much time. There is of course not a hint of anyone seriously proposing any kind of off-world survival colony. I revised my list a few times with an impact as number one for a few years after Chelyabinsk but an engineered pathogen has always returned to me as the most likely extinction level event.
Along with an engineered pathogen and impact event, there is the Earth itself working against our continued survival with climate change and volcanic activity. Zombie fires are one of several feedback loops that could end up killing most of the human race by making it impossible to grow enough food for the soon-to-be 8 billion people on Earth.