incidents of actual terrorism and not people forgetting something in their bag

You're backing yourself into a corner, because you now seem to acknowledge TSA is doing something, you just think its a thing that only applies to "good" people rather than "bad" people.

And your rubric is contradictory. If the TSA stops you with a gun before you get on the plane, you get to say "My bad, please just let me off with warning" or they've failed at their jobs. But if you let someone with a gun onto a plane and then they hijack the plane, they've failed to stop a terrorist. How does a TSA agent stop a terrorist incident on these terms? Is the argument that the TSA is useless because terrorist attacks aren't being thwarted at the moment the individual passes through the metal detector?

air Marshalls and other increased security in the actual plane like hard locked cockpits

Are additional measures that help screen for less-conventional weapons and strategies. But, again, we seem to be using "stopped a terrorist attack" as only happening after it has begun. TSA isn't on board the planes, so there's no way they can ever do the thing you're giving Air Marshals and locked doors credit for.

That TSA as a first-stage screen reduces the number of incidents air marshals and door locks have to prevent as a last resort doesn't seem to matter.