"an immediate, specific, and harmful impact on the national security of the United States."
Yes, and see, they measured it out to be on the order of
"This of course implies that they have no backup system — or at least that the backup are not held for long."
Unfortunately, it proves nothing. The recording systems capture EVERY byte coming along the cables and can be used to REPLAY exactly what data went between two points at any time. This means they can go back and re-examine traffic to find hidden transmissions they overlooked before. They don't throw anything out (specifically stating that they keep anything encrypted forever) and longevity is reported as anywhere from 4 days to several years of 100% of the Internet's activity. Reports vary due to being what the system was capable of at the time, probably. The UT center probably won't be full until your kids are out of diapers and graduating high school.