Well, I'm surprised that Napoli hired him as a new coach. I don't think he is better than Spaletti.
Rudi Garcia was Al Nassr coach, he even failed to win the title of Saudi Pro League. How can Napoli expect Rudi Garcia to win titles in Serie A? However, Napoli may think he is the best option for now since there are not many available coaches to hire nowadays. TBH, I actually still expect that Spalletti can stay for 1-2 seasons more but he has decided to leave.
If Giuntoli joined Juventus, Spalletti seems very clear to replace Allegri.
With this situation, Juventus may keep Allegri for 1 more season, then he will be replaced by Spaletti. But they can sack Allegri soon and replace him with an interim coach.
Rudi Garcia confirmed replace Spaletti position as Napoli head coach although he doesn't has good reputation after sacking by Saudi Pro League teams Al Nassr after failure winning domestic league tittle. But Rudi Garcia have experienced with Serie A because last several years he managed for AS Roma head coach and he not difficult for adapting with Serie A culture.
Massimo Allegri as Juventus position seems secure and not any rumor his position will replace by Spaletti, Juventus is typical teams won't to pay compensation for head coach and sacking Allegri before his contract over until 2027 will spent much money have to pay for Allegri's compensation. Seems not change with Juventus head coach and Allegri will safe his position for upcoming one season later.