Come on, of course it hasn't. Entry requierements haven't changed.
The highschools providing the largest contingents of students to these elite institutions respect neither the national curriculum nor the ban of sorting students by ability without any consequences. Unsurprisingly the two most famous of these highschools are also exempt from the French purely geographical students draft and the places most politician children attend. As usual in France, the rule for all is not the rule for the elite.
In effect, the dumbing down of the national curriculum has just made a system which was already one of the most unequal in Europe even more unequal. But everything is fine. The French system only uses entrance exams and entrance exams are always fair, aren't they?
All countries use entrance exams, it's not something that specific to France and of course there's an inherent unfairness to them, kids from better educated families are always going to get better results. In fact if we were talking about fairness then the disappearance of boarding schools (internats) have actually increased inequality. There was a time when a lot of kids spent the week in boarding schools, for kids from families with less focus on education (like my grandmother who kept telling my mum that she should stop reading because it'd cause her headaches), it removed them from non-optimal environment and put them in situation where they had better access to education. But I'm not sure that's desirable or optimal :)
Both of them looking back thought that repeating a year was a good thing for them, they went from being struggling average/below average students to top of their class and regained their confidence.
I think it really depends, in a place and time where repeating classes is more common, it maybe doesn't really destroy confidence as much whereas in areas where it's rare, then the psychological impact is much worse.
Also, one thing to note, both of them, in their experience with education, saw much better results when children were separated by level than when classes were mixed together for exactly the reason the OP mentioned. A top student can help bring an average student up but when there are both top students and very below average students, the teacher has to make the choice to either focus on the below average student or the top students and neither of those choices are good.
In France, we've had a push toward lowering the overall level and removing any elitism at schools. This has increased the amount of kids who graduate from high school with the Baccalauréat which is needed for university but has lowered the level of kids actually going to university (especially at engineering schools and elite institutions) and reduced the value of a university degree. It's a bit similar to what's happening in the US in STEM and I think it's a bad calculation for the long term competitiveness of the country.