The reason they built it was purely symbolic: that whatever occurred beneath that dome would be open to all the public and that the abuses that started with Herr Hitler all those years ago would never happen again.
But I could see the problem with security.
The security there is tight anyway. They let groups of about 20 into a “holding area”. Then you have to walk through detectors to get to the dome area(if memory serves me correctly). I think I remember 1 entrance and 1 exit to the dome.
Funny story, we were at a Weinachten Market the night before and my husband bought a smoked eel. It got on his gloves which he stuck in his pocket. While we were in the holding area, the smoked eel smell became very apparant and everyone was talking about a smell...!!!
I'm aware of the symbolism, but it still defaced the edifice. Rather like the glass pyramids in the Louvre. .