Real Madrid and Juventus let Ronaldo leaving their clubs even back in those years, Ronaldo still performed very impressively. Now his performance is decreasing to the bottom of his career so it would be no reason for these clubs to sign with Ronaldo. The drama and reaction of Ronaldo in Manchester United make other clubs have to consider more. It would be terrible to have a player who can bring conflicts to a coach and other players.
Ronaldo was frustrated because the coach did not trust him to play, he felt he was still able to give the best performance even though he was not young anymore. Ronaldo really needs playing minutes to maintain his form ahead of the World Cup, he wants to give his best performance because the Qatar world cup is the last in his career.
Ronaldo attitude did not stop other clubs from trying to get him. Just look at Antonio Cassano attitude, his temperament is worse than Ronaldo, but still there are clubs who signed him.
It is guaranteed that Ronaldo could still go to any team he wants, and I mean ANY team he wants, the only thing that would stop would be playing minutes + money. If he accepts to go to a club for what is acceptable amount, and if United sells him for cheap or even free, he could go to Real Madrid and I am not even joking he would play there today, sure it would be hard to place him since they have great players in all positions but they could find a position for him.
The only thing that stopped him during this off season was the fact that United want something in return, and Ronaldo has 20-30 million salary, thats not easy for most clubs in the world.