1. Vieira was better than going forward than Pogba. This is fact. Look at the goals he scored for Arsenal. Look at how much they relied upon him when they attacked with Henry and Pires.
2. Arsenal is one of the highest scoring teams in the PL (if not top 2). As I stated earlier, Arsenal don't have problems scoring; the issue is controlling the midfield and shielding it's defense. Plus, Benzema is a very one-dimensional striker that rarely helps in build up play. Arsenal would be much better off getting a top box-to-box midfielder than a striker.
I wouldn't say Arsenal relied on him for that, this was a team with Henry,Pires,Bergkamp, Ljumberg who between them scored the vast majority of goals. Pires and Ljumberg were good for 30 goals combined in the Invincibles season and there were many times in 2002 specifically where Ljumberg was the clutch player for the team. Even Wiltord and Kanu contributed a lot to those Arsenal teams. Viera was excellent in having a presence in midfield and joining the attacks at times he could stay defensive and then make late runs into the box. Pogba is at his best attacking and for Juventus I have seen Vidal and Marchisio putting in more of a defensive shift.
Arsenal has been outscored by City, Chelsea and United in the past seasons at some point. The gap between Arsenal and a team like City is obvious. Benzema is many things but one dimensional striker he is not, if anything he is the perfect striker to play in a system where Ronaldo and Bale take as many shots as they do. Higuain didn't work out because he is not a good build up player as Benzema. Teams let Arsenal control the ball many times and simply flood the edge of the box, this is how they lose games in England.