I agree. I think that the threat posed by weapons was already fully mitigated on the day of 9/11 when the passengers on the last aircraft tackled the hijackers in spite of the box cutters. All this with no marshal, no sealed cabin door, etc.
The airport screening stuff has been a fiasco that is far more political and psychological than based on actual risk reduction, and the additional cost and time it takes to go through security has cost the economy billions in lost productivity.
I think that the big target at present is actually explosives. I think that C4 resembles the contents of a typical LiIon cell when viewed on a scanner, or could be made to appear quite similar if prepared by someone moderately skilled who had access to (or knowledge of) the scanner and the physics behind it.
Your point about a subway is interesting because we've seen no attacks on subways in the US since 9/11 and even a failed, low-budget attack would result in massive subway line closures and military presence in the subway for months, making it an effective fear-creation attack that would cost a city hundreds of millions of dollars, and potentially much more in lost productivity and mental anguish.
I think the reason we haven't seen those attacks is because so few people would actually want to do them. A can of gas brought into a subway car in a backpack and ignited while the door was closed, combined with an armed shooter using a handgun would cost But on a plane, even a small explosive charge can take the aircraft out of the sky, so I think there is still motive for attackers to try this vector, but if the goal is to make airplanes crash it would be far more effective to use a high powered rifle near an airport and actually shoot at the aircraft. Some airports offer geographical features where this attack would be particularly effective.
That we haven't seen anyone do this also suggests strongly that there is virtually nobody who wants to harm American citizens living in the US, or perhaps that all those who do have extremely grandiose ideas for attacks that are very unlikely to succeed.
The 9/11 attacks were a brilliant way to weaponize aircraft, but after that was used once it becomes a lot harder to use it again. My guess is that if there ever is another large attack it will be something completely different. The first WTC bombing failed, but (similar to 9/11) relied on a very open loophole (fertilizer + van + parking garage under skyscraper). So I think authorities should be focusing on identifying the attack vectors that we are still vulnerable to that haven't been tried.