Low salaries should help the startup scene, not hurt it. Where would you rather create your startup - somewhere where it would cost more to hire or less?
Not to mention, low salaries garantees that not much international talent will relocate in France
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 good fit.
Comparison with SF is hard because it's such a weird place in terms of housing, US healthcare etc. So if you compare to Germany or UK, you still make less in France, but the gap is much smaller than 10 years ago and it's still contracting.
That started before Macron actually. You can find jobs in well-funded startups (instead of the starving startups of 10 years ago happy to raise a million), and more and more international companies (US and others) are now recruiting engineers in France.
Note that if you cross the sea to the UK, London. The salary is double and it's in British pound.