Is the claim that airport security is useless based on known information, reasonings, statistics or other sources?

The current security measures are both excessive and not excessive enough. They inconvenience many people, but those who are willing to lie, cheat, steal, bribe etc can easily get around them.

For example take liquids. A plane was actually damaged with a small amount of liquid explosive that was put together in flight then left under a seat and blew up on the next flight. So we limit the liquids that can go through security. But people who can, say, print out their own label that says Saline Solution can bring a litre of anything through security. Maybe even two litres. So the security won't stop that, but it means people can't bring their half-finished drinks through.

Same with nail clippers, knives, screwdrivers, and so on. Yes, box cutters were used in the 9/11 hijackings. However, getting a box cutter through security is almost as trivial as printing a label. And things are sold after security that are mighty knife-like or that you could sharpen pretty quickly.

Screening all baggage for explosives is sensible, and doesn't slow people down, and isn't decried as security theatre. But the screening of passengers and carry ons, while it may have prevented some attacks, absolutely cannot prevent all attacks (for a variety of reasons; among other things consider a suicide bomber in the middle of a very long security lineup).

And no matter how strongly you screen the passengers and their stuff, what if they have a confederate who works in a restaurant (full of knives) after security, or who can bring in bottles of liquid explosive in their lunch container since they don't go through the same screening, who can drive right up to a plane and in some cases interact with passengers as they get on that plane? With enough money and time, you can meet people who have certain jobs or wait patiently for one of your team to get a certain job.

For this reason people say it's just theatre. It's there to make you feel that flying is safe. "Thanks for keeping us all safe" you say to the officer who rooted through your bag three times until they finally found and confiscated the little flat piece of metal that helps the bag hold its shape, the same bag that went through dozens of screenings before without anyone detecting that always-been-there piece of metal. Do you mean it? Do you feel safer knowing that someone eventually detected the stay in your bag and took it? Perhaps you do. That's what it's for.

What about the "but surely it prevents some attacks?" theory? Well, first, since the checks are widely publicized (everyone knows what they look for and how), a determined attacker can prepare a plan that is likely to get through. What's more, they can even practice -- there are no consequences to having your stuff taken at a security checkpoint, no list of names is kept, you can just try again next week with different stuff until you get through. The 9/11 hijackers did just this with the weaker security in place back then: they travelled week after week the same flights until the security staff got to know them and gave them much less scrutiny than other passengers. They were in suits, they flew this flight every Tuesday or whatever, and they got less searching as a result. While that plan wouldn't work now, others would.

Second, it can actually dilute other measures. If you believe it's not possible to get a knife through security, you may not do anything when you see something that appears to be a knife. The money spent paying people to confiscate nail clippers (not any more; apparently nail clippers are not the threat they once were) is not spent on behavioural analysts or having enough staff to be able to make everyone turn on their laptops or many of the things that happen, for example, at Israeli airports.

When word of the plan to blow up planes with liquid explosives first broke, there was a time on some flights when we could only carry on our passport and one clear bag with maybe a book in it. No laptop bags, no roll-a-boards, etc. Then security procedures got updated and we could have carry ons again. Those carry ons may have threats in them that the current process doesn't detect. But we are allowing them on believing the current process detects everything. Being overconfident in something that doesn't particularly work can blind you to other threats.

Also, before 9/11 people were not crashing planes and killing themselves and everyone else on board using nail clippers, tennis rackets, and whatnot. There were some crashes but the bad guys were not on the plane, they just figured out how to get the suitcase on the plane. There were some hijackings, but the planes didn't crash, they landed somewhere and usually all the hostages were released unharmed. 9/11 and the subsequent shoe bomber, underwear bomber etc changed the attitude of security people and brought in the idea that screening passengers was necessary and effective. It isn't particularly effective. Is it necessary? Only if there's a large pool of people you think want to bring a plane down however they can. (And perhaps you need some of them to be incompetent people who do so on a whim, spur of the moment, without bothering to plan, practice, bribe, etc, since you respond to "but you can get around it" with "What about the people who can't get around it?")

Malicious people can (and have been known to) bomb concerts, the checkin part of airports, subways, and other things, not just planes. Are they even trying to do planes any more? We don't know. Would they switch back to planes if we let up on the security? Maybe. But maybe instead they just try any of the dozens of ways of getting around the security instead. The fact that planes blow up less these days doesn't mean the security stuff is working.