This actually came up after the 2015 attacks in France. Most crowded locations became heavily protected, bags were all checked, etc. The result: a massive crowd in front of theaters, concert and sport venues, etc. Israeli consultants basically summed it up by saying that these procedures were creating massively vulnerable areas with potentially 100+ victims if a guy detonated a bomb in front of the venue.

I remember seeing a documentary showing how in Israeli airport one of the main goals was to always avoid long lines and big groups of people.