Earlier quoted context omitted.
Lots of talented people end up working in another country, especially top talent. Would you rather have a glass ceiling at 40k or start at 100+k in SF? Not to mention, low salaries garantees that not much international talent will relocate in France
France is full of "service" jobs, those are crap and lower the average. Just ignore those and look at the jobs that matter, if you're the kind of developer who would have the skills to relocate to SF. If you're senior, in France, you can get a fairly good salary as a developer. There is definitely no glass ceiling at 40k; you can get more than 50k easily, and above 70k if you have can find the right company with a go…
Note that if you cross the sea to the UK, London. The salary is double and it's in British pound.