If Madrid gets Mbappe they will surely be better, but at the same time Madrid has amazing finances, not spending crazy amount of money like City does, and even when they get a player, they sell some others to cover the difference and earn money to make up for it as well.
This is why I believe that the best thing to do in this case would be making sure that Real has a better future, it is not just about this season, it's about the long term and they are doing fine for the long term without a doubt. They do need a striker and Mbappe would be great for that but no need to get him right now, they can get him next season as well, would be long term but financially smart decision to do that. Other teams just throw money at the issue.
If we compare Real and City, then Real is of course smart money, and City is a brazen cheater who tries to buy everything he can reach. But if you look objectively, I would not say that the future of Real Madrid is guaranteed to be better than that of City - there is simply more money in the Premier League, even if you remove the "dirty" money from there, then I think that the Premier League will still be seriously ahead. That is why Real wants to go beyond its "cage" and create the Super League.