There is a difference though beetween getting to the top of the game and winning a Slam.
French tennis fans would already be over the moon if one of their players just won a Slam, let alone became number one.
There have been many "good" players in the last 30 years who have won Slams, not necessarly great ones like Fed,Nadal,Djoker,Agassi or Sampras.
By the way, i perfectly understand what you mean.
In the past 30 years for sure, you have a point. However at this particular time we're in an era where only great players seem to win slams - the only possible exception being Del Potro. I don't think we can look for structural problems as an answer to why France doesn't have one of these players. I think the structure is actually one of the best in the world and the reason they don't have one of these players is just because they're one of a kind and can emerge anywhere.
Murray is an example - UK tennis has no depth whatsoever, yet through pure luck and circumstance they have one of the best players. If Murray won a slam that would really paper over the cracks of a frankly terrible system in the UK for developing talent.