Think at the present Pjanic is too big a risk. It's unlikely that he has enough to offer as a regular player, and as a backup I imagine he would be rather pointless. That's where you want to give guys like Willock and Lokonga playing time, not someone who's already over the hill.
I could be wrong, maybe he'll go somewhere and have a Pirlo-esque late renaissance, but it seems very foolish to expect something like that to happen. We're not in a position where we can afford to gamble like that either. We need players we're sure can contribute regularly next season.