This is a question i've had for a long time. I looked at the san Fran o-line and those guy's were huge across the board. I think 3 or 4 of them were 1st round picks if i'm not mistaking. The interesting thing i noticed was how athletic they were as well, Altho Matthews got the best of staley at times he did hold his own against Matthews who's fast and strong. That is a line built to be able to run block when it's time to grind a game out and pass block when needed.
Then theres our o-line. Little and slow imo. They are small compared to most nfl lines and you can even tell when you see them on tv which should make them look bigger. I think that's one of the reasons they can't pass or run block right now. To darn small and slow...how many plays did you see where the guard pulled to lead the way for a rb? I saw San Fran do this 4 or 5 times in the course of the game and it worked. Some GM's draft to build good pass blocking attacks and some draft to dominate on the ground but i can honestly look at our line and say wow, these guy's can't run block or pass block. Maybe we need some more brothas on the o-line haha. Jk our "brotha's" can't block either.
There are a couple of factors at work here.
We use a zone blocking scheme. Assignment blocking is generally reserved for short yardage with heavy (numbers, not weight) line packages. We're not trying to grade any roads; it's a finesse approach where you influence a hole rather than plow one. Denver popularized zone blocking with an undersized line and a continuous string of 1000 yard seasons from Terrell Davis, Mark Anderson and Clinton Portis in the 1995 - 2003 time frame. They were so confident of their ability to produce "system tailbacks" they traded Portis after two consecutive 1500 yard season.
Why even do this zone thing? That brings us to the second point. The guys you can get who fit the scheme (smaller, more agile which does not necessarily translate to "fast") tend to be cheaper, and can be developed into decent pass blocking guards. Mostly, it's because they're cheap. If you look at our guards, they were LTs in college, did not project at LT in the pros, and could be gotten without high picks. The fact these kinds of players may have sufficiently quick feet and agility to work in the tight spaces at guard in the passing game, though not necessarily in space which is why they didn't project to tackle, makes them economical. But since they're not going to blow anybody up, the zone blocking scheme is a compromise.
A recent note (at JS Online I believe it was) indicated we're at about $122 mil against the base cap of about $120 mil and adjusted cap of about $128 mil. The adjustments were for the one-time cap penalty against WASH and DAL (which won't be there next year), and the cap carryover (which I don't believe will apply for next year given we're at $122 mil). Buying high priced guards (or centers for that matter) does not seem to be in the cards.
The third point is you need the right kind of running backs. Since there's no assigned hole to run through...you find the hole the line is able to influence...the zone running back needs to be a read and react guy ("one-cut-and-go"). Take the ball, spot the hole (or where you think it is about to develop), cut to it, go. Grant was and Starks is that type of runner. I think the book is open on Benson. He's not played in this kind of scheme, and to my eye he may not be a good fit.
Fourth, little or no Kuhn. MM said in the preseason he was looking to go with more one-back sets, and based on one game he's being true to his word.
As far as I'm concerned, when you're a pass-oriented offense, you need to be able to run the ball in short yardage, goal line, and in the 4th. quarter when you have the lead and are trying to burn clock. On this last point, see Murray's 4th. quarter vs. a gassed Giants D last week. We've been able to accomplish those objectives with the running-back-by-committee the last two seasons. The book is still open on whether Benson can be that guy.
I'm more concerned about the defense...the back 8 lacks a physical presence (Bishop is a BIG loss) and are generally confused, in much the same way they were confused last season. The pass rush is better, which is kinda like what they say about a coat of paint...it covers a lot of sins.