What people forget is that Cesc has started a vast majority of Barca games this season, and will only play even more, and in his favourite position, next season and the seasons to come.
Only if Barcelona don't sign a marquee name to play that #10 role this season.
The real issue is that if Fabregas IS the replacement for Xavi in that position, then Vilanova is going to have to explain why he stuck with Xavi this year. Xavi has 5 goals and 5 assists in the league this year, in a position where he doesn't do any defending. That is really bad. If he had a readymade replacement, and I was his boss, I would be asking him why the hell he stuck with Xavi.
Vilanova is likely to say that Fabregas isn't good enough, and he needs someone better, at which point Fabregas is instantly out of the picture.