I think its a foregone conclusion he will leave but its just a matter of when he will leave. I also dont necessarily think we can't be successful without him if we reinvest the money as we did when we sold Zidane and bought Nedved, Buffon and Thuram. We wont be able to replace pogba with any other midfielder, sure but we can make our team a lot better by signing 3 or 4 players with the money made from Pogba and be better.
Essentially we could use Pogba to replace Tevez long term with Dybala, sign a CM like Allan who will make up for Pogba defensively in the midfield and sign a trequartista who will more then make up for Pogba offensively.
a starting 11 of Dybala,Morata, Firmino, Allan, Marchisio,Vidal, Asa, Chiellini, Rugani/Bon, Licht and Gigi would be good for the long term. Maybe if we have money left over we could get Kurzawa for Left Back... or maybe get Jojo instead of Dybala so we have money for Kurzawa
Either way I think Dybala, Firmino, Allan and Kurzawa would make Juve a lot better for a long time to come then just keeping Pogba would