If Pjanic shows more than Arthur in the next 3-4 years, the deal is not that bad. But if Arthur improves at Juve and plays really well, the deal will indeed look bad. I will follow Arthur and Pjanic and will see what their level over the next few years will be.
I think we need to look at deal separated from each side.
Arthur had 2 years here and he look like he regressed rather than developing, the club has ton of bad deals but it was always new signing not selling. Last one we made a mistake letting go was Thiago (and technically that was another board)
So it can be a decent deal regardless of Arthur performance there tbh.
If Pjanic give us two very good starting level CM, and we plan our midfield rightfully then it is a good deal.
So, if Vidal & Rakitic out this season, it means we have only Pique as veteran in midfield beside Pjanic, so his age and experience is actually needed considering we are very likely to have Pedri/Trincao/Puig in our midfield then we are actually getting younger.
So in such case regardless of Athru performance there, it is a good deal.
OTOH, if we keep old dogs, Pjanic flops it is a bad deal even if Artur doesn't get his ass of the bench there