I would think, for this reason: the supply for normal capacity mags (i.e. 25rd) would be extremely limited. You would only have the supply that was already purchased.
Whereas, even after an AWB, there will be a lot of 30rd 5.56mm mags floating around out there.
You know, it's one thing to be hoarding $15 5.56mm mags right now, it's a bit different proposition to be hoarding mags that cost $30-50 each.
I would think that businesses investing in the 6.8spc platform would work hard, to the extent they can, to fight against a future AWB.