No, I really disagree with most of what you've said here.

The truth is, you can't really know who is gonna perform on not in prépa based on high school results only. I know about people who were really great in high school but failed in prépa, and conversely people who weren't that great in high school and overperformed in prépa. That, plus as I've said this selection tends to be very geographically discriminating, so if what you've said is true it would somehow mean most of the best students would come from Paris. Perceived potential and real potential are two very different things.

In the next paragraphs, I'll talk about the MP prépa because that's what I went through, but I believe it's roughly similar in other prépas.

The prépa matters a lot because, the most prestigious prépa will prepare you specifically for X-ENS, the middle-ranked will prepare you for the Mines and Centrale, and the lowest-ranked will prepare you for CCP. If you're in a low-ranked prépa but you want to prepare the ENS, you'll have to prepare for it by yourself. That's a very, very considerable disadvantage. Of course there's a common program, but the range of what is studied is way, way bigger in the most prestigious prépa - for instance in my prépa in maths we had additional exercises which we were required to know ; when I compared in my school with other students who came from lower-ranked prépa, some of them didn't even know about some of those exercises. It was not only important to know for X-ENS, it made a big part of one of the Centrale exam kinda trivial.

I believe there's also to some extent the fact that the teachers at the most prestigious prépa tend to be the best ones (because those prépa will be attractive for the best teachers) but I'm not sure how this selection works so perhaps I'm a bit wrong here.

Also, yes, the rhythm is obviously way faster so if you're not good enough in this kind of environment you're gonna fail in a prestigious prépa, and if you're good enough for CCP but definitely not X-ENS you'll likely perform better in a middle-ranked prépa. But I really think there's a very sizable portion of students who are good enough for Centrale and Mines and could maybe get X-ENS with proper training and a bit of luck but who are held back by the prépa they're in.