You're right, Mbappe is currently a topic in the world of football all of that means nothing if he never gets the Champions League title and other titles so we all understand his ambition and he knows that only with Madrid will he have the chance to achieve anything. Currently Mbappe is 24 years old, meaning he still has 6 years to pursue a career after reaching 30 years he will think about other things besides career
Mbappe will try to chase titles in Europe and Ballon d'Or too and he will not sacrifice his career in Europe to join Saudi Professional League only because of money he can earn in the nation. He is greedy but he is an ambitious player too and I think his ambition is bigger and more important for him than his greediness and money.
6 years is a very long time, both for the Saudi Professional League as well as the European leagues. Your question regarding the SPL is applicable for the European leagues such as EPL and Ligue 1 as well.
Six years are long if we think of it at the present but if we look at the past, we can see our past six years passed very quickly and it is just yesterday. Career-span of a football player is very short and Mbappe is like other football players, he must to care of his career, his income which he can have though his career and how to achieve best things from titles to personal awards during golden time of his career. To solve all of these and to maximize his career are very challenging for Mbappe.
And in case of SPL, I am expecting this big investments to continue till at least 2030. Because Saudi Arabia is hoping to win hosting rights for the 2030 FIFA World Cup. This is one of the reasons why they are strengthening the SPL, by brining in star players from all around the world. Anyway, Mbappé doesn't have any intention to move to Saudi Arabia for now.
They are a rich nation and has a solid strategy with good support from their government and I believe that they are doing this with a very long term vision, like hosting another World Cup in Middle East after Qatar 2022. They also want to achieve bigger results on field, better than Qatar in the latest World Cup.
To achieve this, they will have to build up a stronger Saudi Professional League and it can not be done after one or two years. So the last few months are beginning of their plan for like one or two next decades.