What do you mean that they hired Ancelotti because they want to win the World Cup? For the Brazilian mentality they take part to the competition in order to win, no matter how bad the team is, the people always expect to win. Ancelotti has nothing to do with that, they hired Ancelotti because, right now, Brazilian managers are quite bad, and so is the team, so they hope that Ancelotti will somehow make a miracle.
I feel that Brazil's problems are also very complex, so Ancelotti becoming the coach of the Brazilian national team may not have such a significant impact. Maybe they can qualify for the World Cup, but beyond that, I doubt they will be able to compete against other big teams. Even against Paraguay, they struggled to score, despite having 74% possession according to the statistics. Not to mention that facing strong European teams will likely be even more difficult. So at this point, it is not just about who is coaching, but more about the basic structure of their game, which is still quite chaotic.