I think it's a great move for him. Juve will win the league every year and reach the quarters and semis at worse even without him. He can give them that finishing touch in the CL while keeping himself fresh for the busy period.
Napoli finished 4 points off last year, 5 points the year before... they got 91 points in the league. Roma eliminated Barcelona from the Champions League, Milan and especially Inter are on the rise. Juve got knocked out in the quarters last year and they were half way out of the competition against Spurs in the round before that.
The gap is closing and Serie A is already on the up, Juve aren't walking to league titles anymore. It was the closest out of the top leagues in the world last season by far. Trying to win the Champions League and keeping the national domination will not be easy, it's a great challenge. After 7 league titles the normal thing is for a slip in consistency and motivation to happen.
Go into it pretending they should be winning 4 league titles and always reaching the CL semis in the next 4 years and yes, it's going to be a big failure. The talk about Juve before these rumours happened was that it was the end of a cycle, let's not forget that.