What, specifically, do you think is going to collapse in the ecosystem that not just kills billions of humans, but actually poses an existential threat to humanity?
To be an existential threat, there have to be less than 5 million humans left (in my mind, at least, and this is probably a really high figure to be 'existential'). This means whatever you propose has to be at least 99.9% deadly.
To put that in perspective, that's three viruses each as deadly as ebola at its worst (~90% deadly) striking humanity one after the other. /That/ would leave ~5-10 million humans alive.
So I'm actually sincerely curious: what do you think is going to happen that will wipe out all of humanity?
To be an existential threat, there have to be less than 5 million humans left (in my mind, at least, and this is probably a really high figure to be 'existential'). This means whatever you propose has to be at least 99.9% deadly.
To put that in perspective, that's three viruses each as deadly as ebola at its worst (~90% deadly) striking humanity one after the other. /That/ would leave ~5-10 million humans alive.
So I'm actually sincerely curious: what do you think is going to happen that will wipe out all of humanity?