Everything dieing is pretty far out on the bell curve. 99% of humans? That's inside the range of reasonable considerations given how little slack we've left in the system.
It comes down to how fast things change. No amount of optimism over comes the fact that you need ~10 megajoules of food and ~3 liters of water on a daily basis. Producing those things for 8 gigapeople involves a lot of infrastructure that took a decent amount of time to build working correctly.
Farmland and logistics hubs in places that no longer get rain water and have run out of aquifers to tap need to be replaced somewhere more fertile. Desalination plants to replace dried up rivers need to be built. None of these can be operated on credit because giving someone a years supply of water after a year of drinking nothing just washes the corpse.
If the numbers shake out that there's a 10% shortfall, which 10% of population volunteers to starve their children? Or do fights break out? How does those fights stay constrained enough to not damage already strained infrastructure exacerbating the problem?
This is not a guaranteed outcome, but if you've seen how wide spread "just in time" logistics has gotten over the last 20 years and think it's impossible, I applaud your optimism.
And also don't want you anywhere near disaster planning.
