The situation as seen from France is odd. Here, elite STEM universities are free, and the very best ones even pay their students. Having legacy admissions would be unthinkable; it's the opposite of meritocracy. And you don't need to "socially" prepare since birth with all the right extra-curricular activities.
Despite this the French system has a very low rate of social mobility, and the overwhelming majority of students in top colleges come from parents within the system.
Now there are good reasons for social background to be sticky (not the least because I don’t buy the tabula rasa theory that your genes account for nothing in your intellectual capacity, and for the part they don’t, well how your parent raised you should matter), but it is implausible that such concentration is a natural outcome (the respective size of elite vs non elites in the country is such that even with a high natural stickiness, you should still see half or at least a third of a top college generation coming from non elite families, I think France is in the Having graduated decades ago and not living in France, I follow it from afar, but it looks like the government is actively trying to eliminate the meritocracy part of the system. By preventing elite high school from selecting their own students, by creating as many parallel admissions to colleges that were purely competitive exams based.
I have mixed feelings about it. One one side I like the idea of a cursus to be purely merit based. On the other side the obsession of the French with education is unhealthy, where your career at 50 is still determined by an exam you sit 30 years before, and this tendency of having clubs within clubs for the elite to run the country with little competence or accountability (again an exam you did when you were 20 doesn’t qualify you for everything) has lead to fuck ups after abuses after frauds. I would prefer a decentralised german style system where (from what I understand) what matters is the student, not the name of the college.