Sergi Roberto is doing pretty good. Yes, he's not regular (yet) but can be. Rafinha has proved his quality too before injury. I think we can safely count on both of them for the future. Others are still unproven on top level, I can agree with that but I believe at least one of them will succeed.
We tried to get younger, WC midfielder like Cesc in the past and it didn't work out. And he was supposed to know our system and adapt quickly. So who can guarantee us that it would be better if we had signed some other younger player last year instead of Arda. Also keep in mind that most of the players wouldn't be willing to sit for 6 months in a year before Euro/Copa.
I think we need some young players this summer and I think Lucho agrees. Enough of the players looking for their last big contract. None of the signings this summer should be older than 25. We need to build for the future.