Traditionally, we're not making many mistakes with sales. I'm a Juve fan for a long time and many times a sale seemed disastrous, just to be proven good or not important. Especially with youngsters (u-21), us fans seem to overreact.
Most of the times, we made sales of important players who, in their new club, weren't even a shadow of their Juve or pre-Juve selves (the calciopoli sales of Thuram, Cannavaro, Zambrotta, Emerson and Vieira, or the sales of Davids, Bonucci, Diego... probably Vidal too). Some times, the players we sold turned out great in their new clubs, but we didn't really regret selling them, since we had adequate replacements (we sold Vieri and replaced him with Pippo Inzaghi; we sold Pippo and we had Trezeguet). In this last group, one might put Tevez-Dybala.
This leaves us with 4 names. The first one is Zidane. We did replace him in the way that after 3 bad/terrible seasons with Zidane, we started winning again with Nedved, Buffon and co. We also got a world record money for him and he was 29. But still, he was Zidane, the best player of his generation.
The other three are imo our biggest mistakes. All of them were 23/24. Zlatan Ibrahimovic became a superstar after Juve. We sold him to Inter, he won them scudetti and then got them a lot of money with which Inter got Eto'o, Sneijder, Milito, Lucio and Thiago Motta and then won the CL. He was a Calciopoli sale, though, so perhaps there was little we could do.
It's the other two who were our greatest mistakes: Thierry Henry and Paul Pogba. It will take more time and space to explain why, but I think it's pretty obvious.
With this being said, although every case is a story of its own, I don't think that after 10 years we will regret or maybe even remember the sales of Cancelo and Kean. It's Dybala I'm worried about. The boy is still 25 and he does have incredible talent.