Cart before the horse regarding Juve. Ronaldo stopped them getting worse.
Truth is Juve had already started regressing before Ronaldo. Compare their 2015 and 2017 squads to what's there now. Inter and Milan were much weaker when the decline started so it wasn't really exposed. However, now that their rivals have caught up Juve don't look like great shakes.
Juve have been relying on freebies to replace quality like Pirlo, Vidal, Marchisio and Pogba. It ain't worked. Ronaldo was the lifejacket. Take him away and there ain't much there.
I don't see the risks you are seeing with Ronaldo. He's an elite six yard box striker, which is something we need given Martial's regression and Cavani's desire to risk quarantine to play for Uruguay. Plus he will frighten opponents, which will create space for others.
Rather than the Zlatan deal, I reckon this will go more like Van Persie in 2012/13.