@AC
Yes - people will demand to know. And yes, there will be many who will say that more should have been done.
But you are assuming a situation that has never been convincingly proven to have occurred in the past. This encryption they want to cripple is not some new super-encryption that has as yet been unavailable. You talk about 9/11 but would the ability to decrypt every American's data - every bank communication, every online purchase, every VPN connection to work?
In a country where it is still possible to buy firearms unregistered, unlicensed and without any background checks - in the majority of states - simply by visiting a gun show - is it really the assertion of these people that the problem is encryption?
Now, I am not saying that restricting the ability to buy firearms anonymously at a gun show would have prevented 9/11 or a similar attack that may occur, but then I am not suggesting the crippling encryption would either!
And that's the point - the assumption behind this push, and behind your hypothetical scenario, is that what is being proposed is going to prevent such attacks. The problem is that there is absolutely NOTHING that has been provided by way of evidence to back this up.
And that's somewhat important here because what is being proposed is HUGE. Not just for the implications for personal freedom and privacy, or for the security of commerce - either of which should be sufficient to nix this - but even just simply when thinking about the technical issues. Combine the three and it's a massive move and one that deserves some kind of justification of why it is worth SO much erosion of privacy and security and so much gimping of the technology*.
Personally, it is my firm stance that such a measure can never be justified but if people are claiming that it is a reasonable move and one that should be taken seriously then it's not outlandish to expect some kind of evidence that it will do any good - let alone out-balance the serious, far-reaching and long-lasting negatives.
* - Off the bat, encryption technologies would be limited to those approved by the government so what happens when those implementations are cracked? Imagine a high-profile vulnerability is discovered in the state-approved encryption - how do to the banks go about updating their systems when to change is to break the law?