Actually, it could. Bethesda's last open world RPG, Skyrim, sold over 20mil total, which is right around the same figures as CoD: AW. The question is simply whether Fallout will experience the same climb in popularity from 3 to 4 as The Elder Scrolls did from IV to V.
Yeah, I don't see it happening...
Skyrim sold that figure in 4 years, AW did it in 1.