One thing that barely gets mentioned in this thread is Fabregas' status at Barcelona. He left England as one of the biggest stars in the league and in Europe adored and revered and also touted to go on and do bigger and greater things than he had to that point in time. Since then, he has become a relative nobody and a spare part who is a mere afterthought at his boyhood club. That, for a player of Cesc's calibre is not living any part of the dream - I am sure he wanted to go back there as a somebody; as someone who would be a celebrated first-teamer contributing to his team succeeding in La Liga and the Champions League. Turns out, they don't need him, haven't even tried to utilize him correctly and couldn't really care less if he left or stayed - and that goes for both the fans and the board. From Fabregas' POV it must be a huge fall from grace and not at all how he imagined his dream return panning out.
Some people here seem to think Fabregas will get his chance when Xavi calls it a day, but I very much doubt he will as he isn't technically proficient or patient enough to do what Xavi does and Barcelona require more from that metronome in midfield than practically anybody else in the team - arguably equal to Messi's contribution, even. They won't give the rein of the team to Fabregas, not with the level he has displayed there to date and I wouldn't be surprised if he himself is aware of that, too.
Contrast that with going to a host of other clubs where he would once again be revered and THE top dog of the midfield again, and I think the door is never going to close regarding his time at Barcelona. A player of his talent cannot see out his best years as some afterthought and spare part and then you add on this being a World Cup year with a direct rival for a slot in the NT moving to Bayern where he will be given the kind of platform Fabregas himself needs to shine, and you've got more than enough reason for Fabregas to want out.
In all the positions Fabregas can play, he now has serious competition for his spot in the NT and all of those players are going to play starring roles for their clubs this season. There is no way Fabregas is not aware of the predicament he is in and unless Del Bosque has given him a private nod saying he's on that plane to Brazil no matter what, he would be a fool to not be considering his future very carefully indeed.