It may be true that our strikers are just getting less clinical each year. There is certainly an argument that Eto'o/Henry was clinical while Ibra/Pedro was just less good, Villa hit a run of awful form the next year, and that last year Pedro was having an off-year and Alexis is not really a striker.
However, these are really good strikers we're talking about. Sure they might have off-years, but chances are one of them would've had a really great year of goalscoring during a three year span, unless something else was going on.
In 2008-2009, Eto'o scored 32 goals and Henry got 26. Since then, no striker has gotten more than 23 goals for Barcelona. During the same period, Higuain got 29 in 2009-2010, Benzema got 26 in 2010-2011, and Higuain got 26 in 2011-2012 while Benzema got 32. It seems odd to me that Higuain and Benzema have each twice hit goalscoring levels in the last 3 years that Ibrahimovic/Villa/Henry/Pedro could not hit. They're not naturally better goalscorers IMO
I'll make the same point in a different way: the best goals/minute ratio for Barcelona strikers during the last three years was, curiously, Pedro in 2009-2010 (by a significant margin too). He got 23 goals in 2638 minutes that year. That's a goal every 115 minutes. Higuain got a goal every 105 minutes in 2009-2010, every 142 minutes in 2010-2011, and every 107 minutes in 2011-2012. Benzema got a goal every 198 minutes in 2009-2010, every 111 minutes in 2010-2011, and every 111 minutes in 2011-2012. Each player had an off-year, but it was the norm for Higuain and Benzema to perform better than the BEST a Barca forward has done in three years. For reference, Ibrahimovic scored every 142 minutes in 2010, Pedro scored every 165 minutes in 2011 and every 212 minutes in 2012, Villa scored every 179 minutes in 2011, Alexis scored every 168 minutes in 2012, and Henry scored every 451 minutes in 2010. Only the WORST season Higuain/Benzema has had (Benzema in 2010) was worse than the next best Barca season after Pedro 2010!
I don't think that this sort of thing can just be due to our strikers not being clinical year after year. Higuain and Benzema are NOT better goalscorers than these guys, but the stats make them look WAY better.
Something else to think about: Higuain and Benzema have averaged a goal scored every 118 minutes in the last three years. If the other main forwards (ie. not including people like Bojan, Jeffren, Tello, Cuenca etc) had scored at this rate, Barca would have scored 15 more goals in 2009-2010, 21 more goals in 2010-2011, and 17 more goals in 2011-2012. This is significant.
This COULD make sense. Here are the problems I have with that logic.
1) Part of them parking the bus more may have to do with the actual change in strategy that I am critiquing the effect of. The offense goes through Messi. When he is in the false nine role, he is in the center of the pitch. Defenses can key up on his attacks quite well by playing very narrow and behind the ball. It is harder to defend narrow and behind the ball when the ball is running through a player who is playing on the wings (Messi pre-2009-2010). Therefore, it was harder to effectively park the bus before. Similarly, teams are able park the bus so much against Barca because Barca's strategy is to play the slow possession game (giving you time to park the bus that you wouldn't have against a counterattacking team). Barcelona play the possession game even more than they did in 2008-2009. I think a good part of this is the fact that it is easier to play a possession game when your attacking focal point is in the center of the pitch. It's much harder to get the ball to a winger and a winger, by nature, has to be more direct than a player in the center. If your objective when in possession is to get it to a player who it is harder to get it to and who has to be more direct once he gets it, then you will retain less possession. In fact, this is the reason, I believe, that they moved Messi to the false nine role. It was easier to get him the ball; they can play less dangerously and keep more possession. The result, though, is also that teams have time to park the bus more. Maybe this makes it harder for the other forwards to score, but that's just my point; the false nine role limits other players.
2) Teams already parked the bus a lot against Barca in 2008-2009. Maybe not as much (for the reasons mentioned above), but that was the main logic behind getting Ibrahimovic after that season. They felt they needed a tall striker to counter bus parking.
3) Teams may have given more space before, but there ARE trade-offs to this. It is not unambiguously better or they would've done it all along. The more you press, the more space you leave behind yourself, for instance. The more you park the bus, the harder it is to score goals themselves, of course. This goes back to the first reason. Why did teams switch their strategy? Was it because they became more tactically aware of Barcelona? Maybe. But I think it has a lot to do with the false nine strategy leading to a less direct and less wide offense, making it easier to park the bus.