It is actually still a mystery why Juventus performed so poorly last season. Of course they will miss Ronaldo, but could it really just be that? I mean they've been champions of Italy for the past 10 years, and they were without Ronaldo's presence. Perhaps there was also a bit of underestimation involved. They are favorites for the coming season, but Inter Milan are certainly no less than Juventus at the moment, I think. But anything can happen in the transfer window.
Injuries were the main problem in Juventus last season imho. Many main players were in injuries that make them unable to play with their best squad every single match. In my opinion, even if Ronaldo stays in Juventus last season, he wont make any significant change. Just take a look at what is happened in Manchester United. Juventus achievement was even better than Manchester United in the local league, while they have equal achievement in UCL.