Such a reliable goalscorer doesn't exist for Barca. Who would you buy (realistically) and replace him with? Villa, a dedicated striker, scored 18 goals in 10/11 and had a dry spell where he scored 1 in 10 in the second half of the season and he was in the peak of his life, 28 years old, coming from a World Cup victory where he was joint topscorer, playing for the best Barca ever. Alexis has already scored more and also in big games (Real Madrid, Atletico Madrid) this season with inconsistent minutes in a worse team and still with room to improve (he's only 25).
I thought the same as you do about goalscoring but then I slowly realized what it means for the team dynamics to play alongside star players like Messi or Ronaldo who are the primary scoring outlets who take 5+ shots per game. Take a look at Benzema at Madrid, he has the same problems, despite playing centrally. It's almost impossible to have Messi scoring 50+ goals in a season with another winger adding 25+ and staying consistent with all the additional workload of tracking back, pressing etc. There can only be 1 main scoring outlet. At Liverpool Suarez' scoring has decreased after Sturridger came back from injury and played centrally. Happens everywhere. Alexis did almost optimal in La Liga this season.
And yes, I agree his overall play can improve. But again. He just turned 25.
Also Messi and Ronaldo have scewed goalscoring statistics. Take a look at Suarez. 31 goals and he is the joint top scorer in Premier League history. You expect a secondary role WINGER to score 25 and stay consistent (Suarez hasn't scored against any top 4 team). It's not realistic.