If Illara is 23, that's good in terms of him having more experience and being more consistant. But on the other hand, he's not been that prominent and has been nowhere near the senior Spain squad so how good is he really?
In terms of Neymar vs Isco, there's two points. First of all, impact is relative to the players you replace. Isco is great but to make an impact he'd have to play better than Ozil for a whole season. Ozil is established as one of the best attacking midfielders in the world both for Madrid and for Germany. For Neymar to make an impact he'd have to improve on the impotent performances of our wingers last season. So Neymar has a much easier task ahead of him.
Secondly, Isco is just another very good player. Neymar is special, in the same way Messi, Zidane, Rivaldo and Ronaldinho are special. He's likely going to contest the Balon D'or at some point in the future. You can see his performances with Brazil, you can see his performances at the Confederations in multiple games including the final how good he is.
You can't compare Xabi and Cristiano with Puyol and Xavi because of the positions they play in. Puyol is a CB and can compensate for less pace with more cautious positioning. Xavi as a CM just jogs around so again doesn't matter too much if he loses speed. Cristiano as a winger relies heavily on his speed and strength, so with age it's harder to do that and crucially it's harder to maintain that for a whole season. And Xabi, he has a lot of defensive duties, so again it's harder for him to rush around trying to cover and harder to maintain that effort for a whole season. Busi will have the same problem when he gets into his 30s.
