The midfield was successful yes. We outclassed Milan with that midfield, we were competing for the title, but because of our lack of depth we lost the title in the fourth quarter of the season, not because our midfield wasn't successful. Us not winning anything that season had nothing to do with the success or lack of success of our midfield, so that argument is invalid.
Also, it's easier to play two in midfield in England (or it certainly was a few years ago) because more teams play two man midfields there than in Italy.
If you're playing a team with three or four in midfield and you're playing with wide men, these wide men will stretch the opposition team, and allow for more space in the middle as the opposition teams midfield will have to open up a bit to defend against the widemen. In that case, your midfield duo will not be outnumbered all the time. Think of United for example, they play a 4-4-2 quite often, sometimes employing that 4-4-2 against teams that play with three or four in midfield ala Chelsea. But they stretch those teams and force them to open up space in the middle because they have to defend against United's wide men.
Or they will play Park narrow on one side or Rooney will play deep to try to stop them being dominated in midfield.
It's pretty rare for Man Utd to play 4-4-2 to two genuine wingers in a big game against a team that isn't just playing 4-4-2.
The main way to succeed with less men in midfield is to avoid playing in midfield, whether that involves long ball or playing the ball round the midfield using the fullbacks to get the ball to the wingers. If you do that, though, it becomes pointless playing a pure playmaker like Pirlo, because you won't be getting him the ball very much.
