I felt the same way about trading up for Decastro - then I realized (a) the guy who coached him in college probably has a better understanding of his talent than I do and (b) you need a trading partner. The only spot where the team traded out of a pick around where Decastro fell to was NE moving into the Bengals pick. And the Bengals wanted a guard (Zeitler) so they needed to stay in front of BAL and SF. But if the team would been interested SF would have had to give up a 3rd and #30 at a minimum. So hypothetically you could have Decastro or Jenkins / Looney / 3rd / 5th Rounder next year.
At this point, I'll take the later. There's a ton of money and picks invested in the oline right now. But adding more bulk and size won't get this team to the next level. The 49ers desperately needed to get faster and get some big play ability out there.
I also agree that the focus of the offense clearly changed towards the end of the season with more passing either out of Gore's injury or Harbaugh's intent. But the concept that this team will only be a smash mouth team I believe is misleading.