I agree with your first premise. They will find a way to take a plane down.
I disagree with your second premise...with a single caveat...they become hired pilots or falsify IDs to act like a pilot.
Hijackers who openly proclaim their intentions will never control the plane for long.
That's another point that I absolutely agree with. The passengers nowadays have the mindset that if the plane is being taken over that it is going to be crashed and that will illicit an immediate response.