Declaring it a "genuine question" does not change the fact that you are baiting people with a logical fallacy.
In all probability, some sort of 9/11 will happen again and this is the question that will be asked.
I like to believe we could prevent another 9/11 without government mandated backdoors.
More importantly, I don't believe we could prevent another 9/11 even with government mandated backdoors.
Government-mandated backdoors seem more likely to enable terrorist attacks than prevent them, honestly.
Given how much government incompetence led to the first one, I'm not sure how more government is going to prevent the next one.
I think the right questions would be: "How can we reduce of orders of magnitude the probability of another 9/11?" and assuming monitor communication being a critical matter, "How could we reduce the probability of communications between terrorist going unnoticed and being able to understand what they are talking about?"
They already had all the pieces to the puzzle when the first attack happened, and they failed to prevent it. Allowing pervasive surveillance of our private communications would throw magnitudes more hay on the stack. So, this is probably not the way to go about it.
Yes. We all need to be prepared to take the risk in return for a free society.