Djourou is fine as a 4th choice CB, and I think it was a good decision to extend his contract. As I have said before though - if the figure of £50k per week is correct, I find it very difficult to belive we couldn´t have kept him for quite a bit less.
Personally I feel fine with our CB options now, especially if Squillaci is replaced by Miquel/Bartley. Because of what we have, I strongly question the rumors that we are after Vertonghen. I don´t particularly like him either, but that is beside the point.
As for Rosicky, I have made up my mind that I think it is better for everyone if he leaves in the summer. Since there reportedly is an offer on the table for him from the club, I hope he rejects it. He is too good not to play regularly, but not good enough to warrant a starting spot at Arsenal IMO. He has proved to be pretty crap as a bit part player as well.
IF the £50k per week for Djourou is true, I don´t even want to think about what we are offering Rosicky for an extension. Without us even strengthening the squad in midfield I cannot see Rosicky being involved in more than 10-15 games next season. There MUST be better ways to use that money.