Ronaldo's position at Juventus was only as a complement, when Ronaldo came, Juventus was at the peak of its glory. Ronaldo came to Juventus at the right time, even without Ronaldo Juventus had won the Italian League in a row. Now just look at Manchester United's performance, if Juventus' success is considered the Ronaldo effect, why at Manchester United Ronaldo cannot improve Manchester United's performance.
To be fair, they were bad even with Ronaldo right before he left. He is a great player there is no doubt about that and I am sure he could play a few more seasons at a top level for sure. However, he is not the type of player that can turn a team into a monster anymore, he is good, but not THAT good anymore.
If you put him on City or Liverpool or Madrid, he could win a lot because those teams are already great, but if you put him on Leipzig today (or even United works for this metaphor) he is not winning any titles because the team around him is not that great. This is his problem, he doesn't pick teams that would already fight for a title, and give them an edge, he just goes to teams that will lose anyway.