The Maginot Line didn’t work either

A limited, centralized defense (fixed security checkpoints, patrol routes, uniformed guards) can be studied, and then avoided or neutralized, freeing the invaders to murder at will.

Citizens, coworkers, free to carry personal firearms and effectively defend themselves, would be everywhere and unpredictable.

If the workers in DC had been free to bring a weapon to work, many would have. Alexis would have been quickly interrupted by people willing to fight.

The voluntary added security wouldn’t cost a dime.