If you move on from Pjanic then you have to change the way you play. Bentancur and Rabiot can do his job for 80% of the matches but when you face the big teams who want to come take the ball from you, they will cause a problem.
Changing the way you play might not be a bad thing, I won't argue those merits, it's hard to say. But simply putting someone else in Pjanic's position might not work the way you think.
Bentancur is surprising us more and more every year though.