The idea that wasting the time of hundreds of millions of people in order to perform pointless "safety theater" is useful is, of course, a delusion. But we humans adore delusion and struggle with reality because our brains aren't evolved to reason from facts. Every time there's a supposed threat, politicians enlist bureaucrats to invent "solutions" that invariably are hilarious for their ineffectiveness. British special forces regularly test airport security and invariably have no trouble smuggling guns, grenades, and other highly lethal weapons onto commercial flights. Furthermore, the idea that 100ml of liquid is magically "safe" while 105ml of liquid is a "threat" is of course utterly risible - as is the idea an 80-year-old woman's scissors should be confiscated. While everyone's time is being wasted on theatrical nonsense, a real terrorist group would simply infiltrate the ground staff (minimal screening, no real security), plant something on the target aircraft, and avoid the whole charade. Or, better yet, target one of the freight aircraft that have no security of any meaningful kind. But bureaucrats and politicians know that the average person is simply too stupid to consider any of these factors and will be comforted by the pointless rituals of security theater. Reminds one of the whole facemask routine, for which there is also no actual evidence of efficacy, just lots of assertions... As for the risk involved in backscatter, it's negligible compared to the risk of driving to the airport even if one amplifies the nominal X-ray risk one hundredfold.