That's definitely an interesting point to consider, and I agree that plays a part, although you can't really ignore money. France basically has a conveyor belt of talented players at the moment, and I'd argue money is the reason they end up in England, as basically every club can pay more than their European counterpart. And because of financial differences and just the sheer quantity of good players, French players have been good value signings compared to a lot of domestic players. There seems to be a similar trend in Germany regarding French talent. So I don't think it's really possible to look at French players in the Premier League without considering financial reasons, because I don't think it's due to cultural reasons.
Is the Seria A really in decline? Juventus clearly are, and Napoli has had better teams in recent years, but overall I feel it's been a steady incline from a worse state. Inter is much improved, Atalanta is an exciting newcomer at the top. Roma and Lazio aren't the most stable , but they've had worse teams. And Milan seem to be on an upward trajectory as well. I can't really judge the rest of the league, but I don't get the feeling that the league is declining. If anything it seems to be slowly recovering, but maybe someone who follows the Seria A more closely than I do will know better.
I think it's easy to come to the conclusion that a league is struggling when the established clubs are going through weaker phases, and sometimes that's true. I think that sort of applies to Spain at the moment, although I think again it just gets amplified when the teams playing in Europe struggle. Other times the rest of a league's level might not really have changed, but it's hard to judge. It's a strange year to really draw any conclusions anyway.