I'm a 49ers fan and I've lived in the Bay Area for nearly my entire life. Had to get that out of the way first, if it even matters.
Garrison Hearst and Barlow are two types of backs that really complimented each other very well in the 49ers offense the past few seasons. They would switch off series to keep the defense off balance and get different types of plays in. I believe that the argument "if Barlow can't beat Hearst for #1, he's no good" is totally bunk. The both played their role in the offense very well.
San Francisco as a team has been in the top10(close to top5) in the NFL in rushing yards (~2250) and rushing tds(~16) for the last few seasons.
The loses at O-line really don't disturb me that much because the 49ers are typically weak in this area anyways. Last year I don't think there were consistent starters on the o-line per se, it was more of whoever was healthiest gets to start that week. But with Hearst, Garcia, Owens, and Streets gone, a major portion of their offense will be new.
I see Barlow making up some of the slack but I figure for the Niner's overall offense to be weaker. Still, if the Niner's total rushing offense declines by 25%, that is 1700yards and 12tds divided between Barlow, Fred Beasley(0 tds in 2 years), and whichever QB is running for his life. Not exactly much competition there.
From watching Barlow play the last few seasons I can truthfully say that he has the raw talent to make it in the NFL as a feature back. This guy hits the hole really hard, and if he doesn't get taken down near the line of scrimmage, he can run past or through the secondary. Imagine a small Jamal Lewis with a better burst. Even if defenses begin to key on him, he has the best blocking FB in the league in Beasley to help him, especially at the goalline.
I think the main gamble you take on Barlow is not on whether he can be the focus of the offense but more on how well the 49ers can do as a team. Will the offense be able to drive the ball and put up points this year? Even if the Niner's are only able to score half as many TDS on the ground compared to last year, Barlow will have 7 or 8 TDs.
I think this is why he is being taken in the 2nd round, because no matter how bad the 49ers are, Barlow is going to score. And if the 49ers are not as bad as people think (bottom 3 in league), then Barlow can put up RB #1 numbers this year.