We tried playing 4-2-3-1 before and we sucked.
As an Allegri era troll you should know this is not true.
Allegri tried 4-2-3-1 at times in the first half of 2012-2013 season and those were the few bright moments of otherwise horrible first months.
He deployed it from start, or he'd switch to it at the end of games when we were chasing goals. That's when El Sha scored nearly all of his goals.
The team found solidity with a 3 man midfield, and 4-3-3, only after De Jong's injury and Flamini's full recovery from his knee injuries.
He should've gone back to 4-2-3-1 in the following season, when De Jong returned and Flamini left, but to be fair the weak, injury-plagued squad wasn't fit for any system.
The decision became much more simple for Seedorf, who was reinforced with 4231 material like Honda, Taarabt, and Essien.
Seedorf's spell wasn't as great as some are making it out to have been, but there's no doubt that the 4-2-3-1 with a low defensive line was more effective than whatever Allegri was attempting earlier that season.
Seedorf's teams were effective for just about 50 minutes during the best period of his tenure. Started out from 20 and reached 50.. after that they tapered off dramatically. His style wasn't really sustainable.
He started off with high-pressing tikitaka "totaalvoetbal" molto champagne calcio, but it failed miserably so he switched to the extreme opposite. Low defensive line, low possession(against minnows) and 3 or 4 lazy soloists and has-beens at the front(Kaka, Taarabt, Balo, Binho).
It wasn't plesant to watch, or always effective, but it was an improvement to Allegri's naive approach to away games.