The Internet was originally designed as a fault-tolerant communications network for the Department of Defense. As it's currently implemented, the Internet would be virtually impossible to "take out" because it has no weak points.
The specific vulnerability linked to probably isn't much of a universal threat, but even if it were such a "weak point," but consider the logistics involved in DOSing the entire Internet. If I have thousands of zombie machines distributed all around the world, as I begin cutting segments, I cut some of my zombies off from the rest of the Internet, rendering them unable to attack. To save links for the zombies, I would have to preserve major telecom infrastructure. If some links are still present, IP works its magic and routes around my DOSing, just as it was designed to do in the case of a cold-war-era nuclear attack, for example.
Another thing to consider is that this particular exploit should have been obvious to anyone with sufficient knowledge of TCP.
