The barrier was never failing. The only reason it failed at all is because we literally murdered Zodiark. What was failing was Venat's hold on Zodiark. The barrier that he himself put into place was just dandy though. In fact, his power was getting stronger as he became more and more whole. He would have kept his barrier going for a while yet. The problem was that no one but Venat knew what was causing the problem to begin with. The Ascians were fighting for their world, but still had no idea what was attacking them.
Edit: Not the Ascians killing everyone was right either. I would have prefered Venat just tell everyone what was up before we had to head into thousands of years of forced mass sacrifice.