From what I've heard, the pillars didn't end up adding to the ambient magic, so much as they multiplied it.
That might sound better, but it meant that on the date when magic was due to go away, it still did, but with a crash rather than a whimper. (zero ambient magic multiplied is still zero magic)
I.E. without the pillars magic would have waned slowly, hitting zero on 12 August 3113 BC
With the pillars, the magic level stayed higher, but still hit zero on 12 August 3113 BC, taking Atlantis with it. (since the city, as built, could not stand without magic)