Arkin stems from a pre-callahan era draft pick. Simple as that. I don't think the transition to a zbs has been particularly kind to any of our players. They've all had their struggles. Smith and Free seem to be doing much better with it this year though.
3/5ths of our offensive line however has been put together in the last 2 years, with 3 new starters this year.
I would say that Callahan gets a pretty high grade, and if Arkin doesn't work it's more a sign of his failure rather than Callahans. But with giving credit to Callahan, you simply have to admit we've given him a lot of tools. 2 first round draft picks, a hall of fame guard... and Free was a highly ranked tackle before Callahan got here. So he has really developed Leary... who I think most had ranked highly outside of his knee injuries (Which are becoming more concerning).
So it boils down to putting some resources into the position. You see our weakest positions and they tend be be ones that don't have any resources there.
Wilcox is the first safety we've drafted in the top 3 rounds in a while, and he was raw, so we can't expect him to be a major player this year.