So you're basically saying without Mbappe Madrid is not winning the champion League? This is a funny stand because they have done it before without Mbappe so why do you think they can't win without Mbappe, when the Mbappe himself has not won it without Madrid?
Man City has a higher chance of winning but they'll win not because Madrid don't have Mbappe, they'll win because they were the better team. Madrid can also win Man City without Mbappe.
Also, Madrid having Mbappe doesn't mean they will definitely dominate. Madrid has had better players than Mbappe before and they have had better squads than they currently have and they did not dominate in those times. Football is not only about stars.
What if Mbappe comes and the team becomes disorganised? Football is a team sports and for a team to succeed, everybody has to do his job effectively.
To achieve a club championship requires collective strength because there is no guarantee that one person can give the team a championship title. Look at how when Lionel Messi, Neymar and Mbappe play as a team, they also have quite a hard time winning the Champions League, even though the strength of these three players is very good on the front lines. The strengths of Real Madrid and Manchester City are both strong now, but if we talk about experience, I tend to see Real Madrid as the strongest candidate at the moment.
Real Madrid has good players in the team and they have achieved many titles with the strength of the existing players. Mbappe's presence might provide much better strength, but there is no guarantee that he will be able to help Real Madrid win the Champions League. The team needs collective strength because there is no guarantee that one person can give the club a championship title.