The second strong end-to-end encryption is implemented, the whole system simply fails to operate. Instead of having a massive data collection system, you now have to go back to more traditional methods that require much more effort to implement - like hacking a target's computer directly. If that happens then the USA will lose its edge in terms of obtaining critical information and that will be a treat to their global dominance....hence the reason they mention national security. Of course, you can't just come out and say this is the reason so they mask it in "fighting sexual exploitation" etc.
The truth is end-to-end encryption and fighting sexual exploitation are not mutually exclusive. We can have both.