The adult response is to acknowledge the trade-offs involved and to learn to live with a certain amount of risk. A free society is always going to be vulnerable to terrorist attacks to some degree. Reasonable policies might reduce this risk by 25%, 50%, 75%, 90%, etc. But they'll never reduce the risk to zero. So the conversation can't be
how do we make sure this never happens again. Instead it must be an analysis of the trade-offs involved in securing against rare events while maintaining the freedom of movement and privacy that a free society requires.
Facial recognition in the UK, the mass phone metadata collection in the US and abroad, stop and frisk in NYC; public support of or indifference to these measures depends on the perception of the trustworthiness of government. That is, for populations that perceive that government as "on their side", the tradeoffs these policies require appear innocuous or beneficial.
So what if the NSA is storing my metadata? They're the good guys trying to catch terrorists. From this perspective the risk of bad government actors is smaller than the risk of terrorism.
So what if cops go hands-on with someone walking down the street? They're finding illegal guns and contraband, and bad guys with warrants. The inconvenience of being stopped is a cost worth paying for safer neighborhoods.
But the fulcrum of government can quickly swing from trust to oppression. There are decades where nothing happens; and there are weeks where decades happen. We cannot keep evil people from attaining power. But we don't have to give them rope with which to hang us. Privacy protections, respect for bodily autonomy, adherence to the rule of law, and the decentralization of economic and political power are all bulwarks against tyranny.