The only reasons I can think of is nuclear war and a second black plague
Nuclear war probably won't wipe out humanity either. Lots and lots of people would die in the countries involved and a few hundred miles downwind of the fallout but other locations would probably only see increased cancer rates. I say probably because while it looks like fears of nuclear winter were probably oversold there's still a fair chance that they weren't and we'd be looking at 5 years with no plant growth after a nuclear exchange. In which case, hey, there's this charity called ALLFED[1] working on how to feed people in the event of a nuclear winter or meteor impact or supervolcano eruption causes a global years long winter. Things like how to operationalize turning dead trees into edible mushrooms at a huge scale.
However, these kids will be dealing with the consequences of extreme climate change in their lifetimes. Lots of people are going to die. Some societies will collapse. It's not going to be easy and having the world leaders today range from offering emotional support and no effective policy decisions to outright hostile to the entire concept of climate change is not helpful.
1: https://en.wikipedia.org/wiki/Special_Report_on_Climate_Chan...
We have enough of a disaster to deal with, there's no need for fearmongering.
For real though I swear half of hacker news must work in oil and gas.
In other words, Y. pestis might still be treatable with modern antibiotics and is well controlled with modern sanitization.
But if MERS-CoV and influenza got together, or NDM-1 genes horizontally transferred to group A strep, you could end up with a highly contagious and highly fulminant threat.