Our entire starting eleven if Vidal starts will have turned 30 next season except Roberto, Umtiti and Coutinho though. I disagree that a very old player is needed. I do agree that someone with his traits can be useful though, like say 25 year old Brozovic from Inter just as an example which someone mentioned.
But what about budgeting?
If you want senior, experience players at top level who are still 'relatively young' isn't easy or cheap. Player profiles who might fit the Vidal mould might be the likes of Pogba or Kante who would be stupid expensive. Brozovic is a good shout but he's never played at the very top or has had any experience in the CL and still could be over our budget.
Look at it this way, over the past year Barcelona has lost Neymar (who falls in the same category as Pogba), Mascherano, Iniesta and Paulinho (if you count him based on his Brazil career). We've added Arthur, Coutinho (who is world class but again has very little CL experience or football experience at the very top level), Dembele, Arthur, Malcom, Lenglet, Alena and might possibly sign FDJ too.
That's a lot of top-level experience to lose and that's a lot of top-level inexperience added.
The Vidal signing makes sense, short term, especially for our midfield.