I won't say that League 1 is not competitive, but it's just that Mbappe won't be able to get much higher career achievements if he's with a team that can't guarantee a title in the UCL or in the european league, whereas if he goes to Real Madrid he gets a guarantee to can win the title in the UCL because everyone knows that Real Madrid is a very solid team that has always been able to get the UCL title because the cooperation between the players is very good compared to PSG even though the salary is only small but after that their popularity will be very high and they will get a future which is very bright if you have succeeded in getting a prestigious title.
we don't need to compare league 1 with La Liga, but just compare the team strength between PSG and Real Madrid, which one is more guaranteed to provide a more beautiful future.
If we talk clearly then we have to said Kylian Mbappé wants to achieve big success which is not possible here in Ligue 1 because here we have no good quality and consistency for him which make things challenging, and he is able to have things as he wanted in last few years we are watching now Ligue one is surely going down and many big clubs are also suffering badly with their consistency is also not good, and performance level is also badly down which are making life not easy for the mega star like Mbappé so just because of this now it's good time for him to leave this all and enjoy which he will have better challenges and success as well even money will be not like he is having from PSG, but he can achieve many milestones which are not possible here.
Here I agree, never need to compare leagues or clubs as they are performing in completely different conditions and also available sources and their system are also completely different, so things like these are not comparable but still players can manage as they wanted for the better future.