I'm a huge fan of Thiago, one of my favourite players. Earlier this summer I was adamant we should sign him. And indeed I still think he's better than any of our mids bar Busquets. (Coutinho at this point I'm classing as a FW). However I've high hopes for Arthur, heard good things about Puig and Alena (the latter admittedly I've seen nothing in the handful of mins he's had to suggest he can be top quality; hell, Foden looks the better player and he's like 2 years younger; but hopefully I'm wrong and he'll make it); that added to his price and injury history and I'm inclined to agree that the time to sign him has passed.
That is assuming, however, that Valverde gives the young guys a chance. Meaning Puig has to play every Copa game unless we get Real, maybe even some league games against lesser opponents, and Arthur needs to be getting minimum 20, 25 starts. I'm also hoping we can sign De Jong, which would make more sense long term. However if none of that happens, if we end up the majority of games 442 with Rakitic rm and Vidal in centre, well then I'm sorry but the decision not to sigh Thiago is insanity. So yeah, in conclusion I'm hopeful but skeptical.