The purpose of the "focal points" (the cuendillar discs) could have been to "absorb" the weaves of the sealing (without changing how this energy is organized into weaves), making them impossible to attack in any way the AOLers knew of. Thus, if the sealing succeeded, it would have been theorically impossible for anyone, human or DO, to destroy it. Or so they thought.

The DO's energy seems to corrupt everything it touches in Creation. Whether it's the taint on saidin which the seals likely absorbed during the backlash, or the DO touching the seals with the TP that eventually got absorbed little by little by the cuendillar discs, it sounds like the explanation why the discs are crumbling is that the DO's energy has managed over time to alter the structure of cuendillar. As of now, it's no longer cuendillar, whatever the matter has turned into (the discs no longer absorb energy, for one thing).