For those in those French "Ivy league" it makes sense to stay in a non-meritocracy / credentials based company then, but for all other engineering schools, it's a better deal to get a 6 figures salary job in the US, where it's much closer to a meritocracy than in traditional French companies.
In US tech you certainly see some Harvard, Stanford, mit graduates, but also a lot of non ivy league colleges with jobs such as software engineer, program manager, management, etc. One extreme example is Satya Nadella who got his MS from University of Wisconsin. Even if is undergrad degree was prestigious is India, it doesn't have the ivy League "power" similar to a X/Centrale/Mines in France or Harvard in the US in traditional jobs.
However, if you're from these schools but have nothing except your hard work to sell, you're better off in UK/USA (and your alumni network definitely extends there)... but you're likely a Parisian with friends/family/housing in Paris, so there are strong incentives to stay in the place you already are. Your school name alone will afford you a career decent enough for little effort.
For someone from any other school who ain't Parisian, there is no incentive to emigrate to Paris. The opportunities are much better in the UK/USA to the point of insanity, and it may even be easier logistically (it was for me).
For an European. Why the hell would he go to Paris? It's not even worth if for French people who ain't already in Paris.
I am dubious because as mentioned by hiq above, you would need to wait much more than 2 years to figure out who are the "best". Another reason I am dubious is that working in France or at least in a French company is still the best way to climb the hierarchy fast when you're coming from X/Central Paris/ Mines de Paris/Telecom/etc... except for a few specific fields (finance comes to mind).