Seriously, how can anyone think Ronaldo doesn't perform good on Manchester United? he's on top 3 scorers and shoots, while his club is struggle with internal problem and have a bad record. This mean Ronaldo already carrying Manchester United to become better, but as we know football as a team sport, can't only relying single player.
Looking how Maguire play style on Manchester United, I'd think he's a Central Attacking Midfielder rather than a Central Back.
Ronaldo is old and his performance decreases. He still can score many goals but can not cover a whole team. Manchester United just expected too much from Ronaldo while their whole system did not work well in the last season. I think it is one of reasons why Ronaldo felt disappointed and want to leave.
Same for Maguire. The performance of Maguire in Manchester United and England are very different. I know Maguire was overbought by Manchester United but his quality is not too bad. Of course I also know his quality is not world class but still usable. It is just Manchester United take all confidence of Maguire away.
About Ronaldo, if he stays, the whole system must work better than in the past season. If their system works better, Ronaldo can play better, score more goals for them.