Steve, the "21st century" comment may be similar in form to the infamous WT statement about the 20th century that was later revised, but utterly different in content and probability, I would suggest.
All generations view their own time as unique and are aware of new risks. But the invention of the nuclear bomb was a game changer. Humans now have the ability to end our planetary civilisation in minutes. We have come incredibly close to doing so a number of times. It would be foolish to think we will not come close again, or that we will avert the worst every time. It's just one of those things better not to think too much about I suppose, but nevertheless very real.
Astronomer Martin Rees gave various scenarios and reasons for suspecting humans would not last another century, but by far the most worrysome to me is nuclear conflagration.
https://www.amazon.co.uk/Our-Final-Century-Civilisation-Twenty-first/dp/0099436868