If not now then when? He's 27. The prime of his career. And the only reason Juventus need him is because they play 3 cms, one in his 30s, so they need a good backup and he is that. If they sell him it'll be because his injuries are a concern and he doesnt want to be a backup.
If he was the player of a few seasons ago, without the injury issues and with better form you'd expect that he, Vidal and Pogba would be Juventus' priority building a side around those 3 in midfield. Instead its still grand daddy Pirlo and they might let Marchisio go.
If we could get him for £20-25 million, he doesn't have any more injury issues and recaptures the form of 18 months ago then it'd be a great signing but when you're signing a player out of form, playing a squad role and with injuries then its a gamble. Thats why he's worth nowhere near what he would have been at his best a while ago