This is cynical past the point of healthy. There are plenty of sectors that would be revolutionized if smart people focused their attention on them.
You don't have to cure cancer; that's absurdly binary and based on fame. You can do less sensational things like invent a device that improves patients' lives using modern robotics and sensors. You can design a system for Alzheimer's patients that incorporates your knowledge of big data sets. I can't dream of all the things to improve, but your domain knowledge is probably tremendously useful in all kinds of fields.
In addition, even "cat picture project" technology can sometimes be used to implement "world changing" technology. I think "do what you love doing and always challenge yourself" still generally applies here.
That's the rationalization that smart people tell themselves so that they can sleep at night.
Accidents can always happen but if you're writing PHP to tag cat pictures, no, it's not going to do anything. But keep telling yourself that if it helps you sleep.
You don't have to cure cancer; that's absurdly binary and based on fame. You can do less sensational things like invent a device that improves patients' lives using modern robotics and sensors. You can design a system for Alzheimer's patients that incorporates your knowledge of big data sets. I can't dream of all the things to improve, but your domain knowledge is probably tremendously useful in all kinds of fields.