Cameron is at least close to right. The 2010 Beltre was monstrous, and regardless of Youk's quality at third (which the writer underestimates), Youk is not as good as Beltre at third, and the same could be said of A-Gon and Youk at first. A-Gon's bat will probably end up being better, but it's not exactly clear-cut that a 2011 A-Gon is better than a 2010 Beltre.
However, this is obviously not the relevant decision that the Sox had to make. A 2011 A-Gon will be better than a 2011 Beltre, given a lot of the luck that Beltre had in his 2010; and a 2011-2014 A-Gon will definitely be better than a 2011-2014 Beltre.