Do you people ever watch Barca?
Alexis/Villa/Pedro have tried countless times to take on their man, and 99% of the time they fail. It is very simple, they are bad dribblers. One good defender can take care of them. In the Supercup Cam Nou Classico, Alexis was one on one with Coentrao at least 5 times, not once did he manage to beat him. Lots of times Alexis/Pedro are in a goalscoring position, usually they waste the chance. Alexis, even when in good position to shoot, is reluctant to do so, and when he does, he doesn't score. Of course when Iniesta/Cesc has Messi and Alexis within passing distance, they will prefer Messi, as would I, due to the simple fact that Messi has a vastly higher probability of turning that pass into a goal or assist. ALexis and Pedro are failing the team, not the other way round. Messi is too good, they are no more than decent. I hope something happens quickly, and turn me into a fool, but it seems ever unlikelier.
A few seasons ago, it was a great team effort, because Eto'o and Henry were scoring a good % of the chances created by Messi, Xavi, Iniesta and Alves. Pedro and Alexis are lesser players than Eto'o and Henry I'm afraid.
As for Xavi, people have to realize that he is approaching 33. He is not as athletic or as potent as when he was 28 in 2008/09 (his best year for me, along with maybe 2010) He has less energy now, and therefore is more difficult for him to approach the opposing box, and provide the killer pass etc...
People who say these things are in my mind excessively protective of Pedro (because he is a hard-working canterano) and also very protective of Alexis who is also a very nice bloke with all the good intentions in the world, but are do far falling short. Another category of people that ascribe to you theory, are those that do not want to admit the full extent of Messi's magnificence, and are trying to say that he scores so unbelievably much because Barca are designed solely for that purpose.
My word fellow Cules, if we were to miss Messi even for a few games, then you would see how hopeless our attack would be. I hope that never comes to be.