Unfortunately, we humans are imperfect. We work based on what we know, and what we think we know, and we make the decisions and assumptions that seem logical based on that.
However, we cannot truly know what is and is not correct. If we are told by someone we trust that something is true, we will often believe it is, regardless of whether or not it actually is true. This is base human nature. We are incapable of investigating the truth of every matter, either because of time constraints, or because of our physical ability to determine the truth of those matters.
This means that for over 95% of the things we "Know," we don't truly know. We're merely believing what we have been told.
If we had the magical ability to always know the truth, we would be physically incapable of truly dying as a species. Up until the point that our universe dies, we will live. Of course, we don't have the magical ability to know the truth of all matters, so we will likely not live to the end of the universe, and the cause of our downfall will almost certainly be our ignorance, willing or otherwise.
Yes, the tendency of humans to believe things that aren't true will lead to our downfall. This will be exasperated by the spread of the internet and social media throughout the world. In our day of instant gratification and sharing, people don't care to look things up or to fact check. They take things that they believe, and they build on those things with 'facts' that support their beliefs, whether they are true or not. This moves so much faster with social media.
Yes, people tend to believe in mysteries and explain them as signs of some upcoming Dooms Day. How many Dooms Days did we have in this decade? If I can count well there were four or five ''the end of the world'' scenarios. And we are still alive and kicking, right? Many smart people have used the naivety of the masses to earn a big money. Are they to be blamed? Definitely not.
It often happens that civilizations face unnecessary death and hardships because they cling to unfounded superstitions and beliefs. Some cultures have low life expectancy because they reject modern medicine. Just as rejecting advancements in medicine can lead to early death, a civilization that puts too much faith in science to solve every problem can be doomed.
Donald Trump is a good example of this willful tendency to ignore the truth. If he were to become President, it would be a disaster. It's not just him, it's also those who believe in him. Brexit is also a good example. People ignored that it could actually happen, didn't vote and now Britain is out of the EU. Vaccines can save lives, global warming is real. Ignoring scientific truths, and medical facts because you don't want to believe them can harm us.
Life can be difficult for some and is very hard to others. Sometimes all they have is what we may consider believing in false things.
If we point out to people that the truth is they will die, eventually everything they ever did will be forgotten, our society and species will go extinct, it can become depressing. If we stayed in the realms of today, we could say the truth is statistically they contribute so little to society they aren't significant in any way without a few exceptional individuals. If we look at more philosophically debated terms we could say the "truth" is there is no evidence of a God or any purpose for their lives. If we think of extraterrestrials we could say the truth is we are alone and always will be (A separate debate for sure) or we aren't alone and they are so advanced our species is insignificant in comparison and ignored.
These harsh truths can shatter realities. Like taking the joy of Christmas away from a child telling her that Santa is not real, that there is no tooth fairy or Easter Bunny. Nothing is gained exposing her to this truth. This can cause a wide adoption of nihilistic beliefs which often results in either complete apathy or hedonism.
An apathetic society will not progress and surely die out faster with no more advancement because why bother? A hedonistic society will eventually destroy itself with no real goals or order.
It may not be the truth, but having beliefs make people feel important, it makes them have hope, it makes them want to try and live their lives and build a better future for tomorrow. If we didn't have this hope and self-importance we would already be extinct.