The fact Juve went from Buffon to Szczesny is an example of how, over time, your squad has been downgraded. You can take a number of players in the Juve squad, compare them to the players they replaced, and make a similar comparison.
The idea that Ronaldo has held Juventus back, which seemed to be what you were implying, just does not stand up to evidence. The quality of Juventus' squad has been getting worse and worse in recent years. As I said before, without Ronaldo's goals, Juve would have fallen behind Inter earlier than you did.
Sarri might be a good coach but he was not a good coach for Juventus' squad. It was obvious, to everyone, that Sarri's deliberate, passing style would not suit the Juventus team built to work with Allegri's reactive football. The appointment of Sarri is just another example of the bad decision making at Juventus in recent seasons. If you have a squad put together for a particular style of football, and you are either unable or unwilling to make major investments so the squad can go in a different direction, it makes no sense to get a coach who doesn't fit your players. Juve would have done a lot better finding a more pragmatic coach who could work with what he had. Rather than an idealist like Sarri.
Tired in what sense? Is he massively more immobile than other Premier League forwards? Yes, he picks his moments. That's because he is older. Nobody is denying that age has had some impact on him. However, when the moment calls for it he can still be as dynamic as most strikers in the top flight. I mean, to be frank, to the eye, there's little difference between Ronaldo up front and Martial. Martial isn't exactly busting a gut to close down defenders either. Its just Ronaldo can do everything Martial does better.
I remember the last days of Rooney's career at United, when he looked half the player he was at his peak. To me that's what a player looks like when their legs have gone. Compared to that Ronaldo looks like he's got years yet to go.